feat(KB-617): add diff viewer tab to task detail modal

- Add modifiedFiles and baseCommitSha tracking during task execution
- Create GET /tasks/:id/diff API endpoint for file list and patches
- Build TaskChangesTab component with expandable file diffs
- Integrate Changes tab into TaskDetailModal for in-progress, in-review, and done tasks
- Add database columns and types for diff tracking
- Update executor to capture modified files during agent sessions

packages/dashboard/app/api.ts

@@ -1868,3 +1868,14 @@ export function fetchProjectTasks(projectId: string, limit?: number, offset?: nu
 export function fetchProjectConfig(projectId: string): Promise<{ maxConcurrent: number; rootDir: string }> {
   return api<{ maxConcurrent: number; rootDir: string }>(`/projects/${encodeURIComponent(projectId)}/config`);
 }
+
+/** Diff information for a task */
+export interface TaskDiff {
+  files: string[];
+  diffs: Record<string, { stat: string; patch: string }>;
+}
+
+/** Fetch diff information for a task */
+export function fetchTaskDiff(taskId: string): Promise<TaskDiff> {
+  return api<TaskDiff>(`/tasks/${encodeURIComponent(taskId)}/diff`);
+}

packages/dashboard/app/components/SetupWizard.tsx

@@ -396,7 +396,7 @@ export function SetupWizard({ isOpen, onClose, onProjectCreated, onRegisterProje
               <button
                 className="btn btn-primary"
                 onClick={handleValidate}
-                disabled={state.isValidating || state.validationError}
+                disabled={state.isValidating || !!state.validationError}
               >
                 {state.isValidating ? (
                   <>

packages/dashboard/app/components/TaskDetailModal.tsx

@@ -13,6 +13,7 @@ import { ModelSelectorTab } from "./ModelSelectorTab";
 import { PrSection } from "./PrSection";
 import { TaskComments } from "./TaskComments";
 import { MergeDetails } from "./MergeDetails";
+import { TaskChangesTab } from "./TaskChangesTab";
 
 interface ModelSelection {
   provider?: string;
@@ -105,7 +106,7 @@ export function TaskDetailModal({
   addToast,
   githubTokenConfigured,
 }: TaskDetailModalProps) {
-  const [activeTab, setActiveTab] = useState<"definition" | "activity" | "agent-log" | "steering" | "comments" | "model">("definition");
+  const [activeTab, setActiveTab] = useState<"definition" | "activity" | "agent-log" | "changes" | "steering" | "comments" | "model">("definition");
   const [attachments, setAttachments] = useState<TaskAttachment[]>(task.attachments || []);
   const [uploading, setUploading] = useState(false);
   const [dependencies, setDependencies] = useState<string[]>(task.dependencies || []);
@@ -671,6 +672,14 @@ export function TaskDetailModal({
             >
               Agent Log
             </button>
+            {(task.column === "in-progress" || task.column === "in-review" || task.column === "done") && (
+              <button
+                className={`detail-tab${activeTab === "changes" ? " detail-tab-active" : ""}`}
+                onClick={() => setActiveTab("changes")}
+              >
+                Changes
+              </button>
+            )}
             <button
               className={`detail-tab${activeTab === "steering" ? " detail-tab-active" : ""}`}
               onClick={() => setActiveTab("steering")}
@@ -703,6 +712,8 @@ export function TaskDetailModal({
                 validatorModel={getValidatorSelection(task)}
               />
             </div>
+          ) : activeTab === "changes" ? (
+            <TaskChangesTab taskId={task.id} worktree={task.worktree} />
           ) : activeTab === "steering" ? (
             <SteeringTab task={task} addToast={addToast} />
           ) : activeTab === "comments" ? (

packages/dashboard/app/styles.css

@@ -11812,3 +11812,134 @@ html .column.drag-over * {
 [data-theme="light"] .gm-load-more:hover {
   background: rgba(0, 0, 0, 0.03);
 }
+
+/* ── Task Changes Tab Styles ─────────────────────────────────────────────── */
+
+.task-changes-tab {
+  padding: 16px;
+}
+
+.changes-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  margin-bottom: 16px;
+}
+
+.changes-header h4 {
+  margin: 0;
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  font-size: 14px;
+  font-weight: 500;
+}
+
+.changes-file-list {
+  border: 1px solid var(--border, #30363d);
+  border-radius: 8px;
+  overflow: hidden;
+}
+
+.changes-file-item {
+  border-bottom: 1px solid var(--border, #30363d);
+}
+
+.changes-file-item:last-child {
+  border-bottom: none;
+}
+
+.changes-file-item.expanded {
+  background: var(--bg-secondary, #161b22);
+}
+
+.changes-file-header {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 10px 12px;
+  background: none;
+  border: none;
+  width: 100%;
+  text-align: left;
+  cursor: pointer;
+  color: var(--text-primary, #c9d1d9);
+  font-size: 13px;
+  transition: background 0.15s;
+}
+
+.changes-file-header:hover {
+  background: var(--bg-hover, #1f242c);
+}
+
+.changes-file-toggle {
+  display: flex;
+  align-items: center;
+  color: var(--text-secondary, #8b949e);
+  flex-shrink: 0;
+}
+
+.changes-file-status {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 20px;
+  height: 20px;
+  border-radius: 4px;
+  font-size: 11px;
+  font-weight: 600;
+  flex-shrink: 0;
+}
+
+.changes-file-path {
+  flex: 1;
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+  font-family: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, monospace;
+}
+
+.changes-file-stat {
+  color: var(--text-secondary, #8b949e);
+  font-size: 11px;
+  flex-shrink: 0;
+  margin-left: 8px;
+}
+
+.changes-file-content {
+  border-top: 1px solid var(--border, #30363d);
+  background: var(--bg-primary, #0d1117);
+}
+
+.changes-diff-patch {
+  margin: 0;
+  padding: 12px;
+  font-size: 12px;
+  line-height: 1.5;
+  overflow-x: auto;
+  white-space: pre;
+  font-family: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, monospace;
+  color: var(--text-primary, #c9d1d9);
+}
+
+.changes-diff-patch code {
+  background: none;
+  padding: 0;
+}
+
+/* Syntax highlighting for diff */
+.changes-diff-patch .diff-add,
+.changes-diff-patch [data-prefix="+"] {
+  color: #3fb950;
+}
+
+.changes-diff-patch .diff-del,
+.changes-diff-patch [data-prefix="-"] {
+  color: #f85149;
+}
+
+.changes-diff-patch .diff-hunk,
+.changes-diff-patch [data-prefix="@@"] {
+  color: #58a6ff;
+}

packages/dashboard/src/mission-routes.ts

@@ -51,20 +51,29 @@ function validateUuid(id: string): boolean {
   return /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i.test(id);
 }
 
-function validateMissionId(id: string): boolean {
-  return /^M-\d+$/.test(id);
+function validateMissionId(id: string | string[]): boolean {
+  const str = Array.isArray(id) ? id[0] : id;
+  return /^M-\d+$/.test(str);
 }
 
-function validateMilestoneId(id: string): boolean {
-  return /^MS-\d+$/.test(id);
+function validateMilestoneId(id: string | string[]): boolean {
+  const str = Array.isArray(id) ? id[0] : id;
+  return /^MS-\d+$/.test(str);
 }
 
-function validateSliceId(id: string): boolean {
-  return /^SL-\d+$/.test(id);
+function validateSliceId(id: string | string[]): boolean {
+  const str = Array.isArray(id) ? id[0] : id;
+  return /^SL-\d+$/.test(str);
 }
 
-function validateFeatureId(id: string): boolean {
-  return /^F-\d+$/.test(id);
+function validateFeatureId(id: string | string[]): boolean {
+  const str = Array.isArray(id) ? id[0] : id;
+  return /^F-\d+$/.test(str);
+}
+
+/** Helper to extract string from Express param (handles string | string[]) */
+function paramString(value: string | string[]): string {
+  return Array.isArray(value) ? value[0] : value;
 }
 
 function validateTitle(title: unknown): string {

packages/dashboard/src/routes.ts

@@ -1841,6 +1841,82 @@ export function createApiRoutes(store: TaskStore, options?: ServerOptions): Rout
     }
   });
 
+  /**
+   * GET /api/tasks/:id/diff
+   * Get detailed diff information for files modified during task execution.
+   * Returns: { files: string[]; diffs: Record<string, { stat: string; patch: string }> }
+   */
+  router.get("/tasks/:id/diff", async (req, res) => {
+    try {
+      const task = await store.getTask(req.params.id);
+      
+      // Only tasks with worktrees can have diffs
+      if (!task.worktree || !existsSync(task.worktree)) {
+        res.json({ files: [], diffs: {} });
+        return;
+      }
+
+      // Use stored modifiedFiles if available, otherwise compute on-the-fly
+      let files = task.modifiedFiles;
+      
+      if (!files || files.length === 0) {
+        // Fallback: compute files using git diff
+        try {
+          const baseRef = task.baseCommitSha ?? "HEAD~1";
+          const output = execSync(`git diff --name-only ${baseRef}..HEAD`, {
+            cwd: task.worktree,
+            encoding: "utf-8",
+            timeout: 5000,
+          }).trim();
+          files = output ? output.split("\n").filter(Boolean) : [];
+        } catch {
+          files = [];
+        }
+      }
+
+      if (files.length === 0) {
+        res.json({ files: [], diffs: {} });
+        return;
+      }
+
+      // Compute diffs for each file
+      const diffs: Record<string, { stat: string; patch: string }> = {};
+      const baseRef = task.baseCommitSha ?? "HEAD~1";
+
+      for (const file of files) {
+        try {
+          // Get stat for this file
+          const stat = execSync(`git diff --stat ${baseRef}..HEAD -- "${file}"`, {
+            cwd: task.worktree,
+            encoding: "utf-8",
+            timeout: 5000,
+          }).trim();
+
+          // Get patch for this file
+          const patch = execSync(`git diff ${baseRef}..HEAD -- "${file}"`, {
+            cwd: task.worktree,
+            encoding: "utf-8",
+            timeout: 10000,
+          });
+
+          diffs[file] = { stat, patch };
+        } catch (err: any) {
+          // Log error but continue with other files
+          console.warn(`Failed to get diff for ${file}:`, err.message);
+          diffs[file] = { stat: "", patch: "" };
+        }
+      }
+
+      res.json({ files, diffs });
+    } catch (err: any) {
+      if (err.code === "ENOENT") {
+        res.status(404).json({ error: `Task ${req.params.id} not found` });
+      } else {
+        res.status(500).json({ error: err.message || "Internal server error" });
+      }
+    }
+  });
+
   /**
    * GET /api/tasks/:id/workflow-results
    * Get workflow step execution results for a task.