FN-6675: add GitHub close-time backfill action

Add a Command Center operator path to backfill exact GitHub source-issue close times.

- Add a dashboard API client for the paginated GitHub source-issue closed-at backfill endpoint.
- Add a GitHub Command Center button, progress/error/result state, and styling for manual backfills.
- Cover backfill success, empty, error, and pagination behavior with area tests and document the operator flow.

Files changed:
 .../fn-6675-github-closed-at-backfill-dashboard.md |   5 +
 docs/dashboard-guide.md                            |   4 +-
 docs/storage.md                                    |   2 +-
 packages/dashboard/app/api/legacy.ts               |  25 ++++
 .../components/command-center/CommandCenter.css    |  39 ++++++
 .../components/command-center/areas/GithubArea.tsx | 117 ++++++++++++++++-
 .../command-center/areas/__tests__/areas.test.tsx  | 140 ++++++++++++++++++++-
 7 files changed, 326 insertions(+), 6 deletions(-)

Fusion-Task-Id: FN-6675

Fusion-Task-Lineage: 09e43b60-b9db-421f-a131-740a0d3164fe

packages/dashboard/app/api/legacy.ts

@@ -7196,6 +7196,14 @@ export interface KillVitestResponse {
   pids: number[];
 }
 
+export interface GithubSourceIssueClosedAtBackfillResult {
+  scanned: number;
+  filled: number;
+  skipped: number;
+  errors: number;
+  hasMore: boolean;
+}
+
 export function fetchSystemStats(projectId?: string): Promise<SystemStatsResponse> {
   return api<SystemStatsResponse>(withProjectId("/system-stats", projectId));
 }
@@ -7206,6 +7214,23 @@ export function killVitestProcesses(projectId?: string): Promise<KillVitestRespo
   });
 }
 
+/**
+ * FNXC:GithubSourceIssueBackfill 2026-06-18-19:20:
+ * Thin client for the FN-6674 manual source-issue closed-at backfill endpoint. Callers own bounded pagination until `hasMore === false`; this helper keeps the GitHub lookup in the explicit operator action path and out of analytics/render-time data loading.
+ */
+export function apiBackfillGithubSourceIssueClosedAt(
+  options: { offset?: number; limit?: number } = {},
+  projectId?: string,
+): Promise<GithubSourceIssueClosedAtBackfillResult> {
+  return api<GithubSourceIssueClosedAtBackfillResult>(
+    withProjectId("/git/github/backfill-source-issue-closed-at", projectId),
+    {
+      method: "POST",
+      body: JSON.stringify({ offset: options.offset, limit: options.limit }),
+    },
+  );
+}
+
 /** Fetch unified activity feed */
 export function fetchActivityFeed(options?: FeedOptions): Promise<ActivityFeedEntry[]> {
   const params = new URLSearchParams();