feat(KB-617): add task diff Changes tab and backend support

- Add core task fields for base commit and modified files to support diff computation
- Implement dashboard and API wiring for task changes, including Task Detail Changes tab integration
- Update executor flow to capture modified files during execution for accurate diff display
- Add tests and changeset covering the diff viewer feature

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/styles.css

@@ -12535,3 +12535,134 @@ 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/routes.ts

@@ -2240,6 +2240,82 @@ 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.