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/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.