feat(KB-617): add Changes tab to task detail modal for viewing file diffs

- Add baseCommitSha field to tasks for diff computation baseline
- Track modified files in executor and store in database
- Add /api/tasks/:id/diff endpoint to serve unified diffs
- Add getTaskDiff() API client function in dashboard
- Create TaskChangesTab component with file list and diff viewer
- Integrate Changes tab into TaskDetailModal with proper styling
- Add changeset for the new diff viewer feature

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.