fix: use GITHUB_RUN_NUMBER for Forgejo job URLs and pin Bun version in CI

Forgejo uses the per-repo sequential run index (GITHUB_RUN_NUMBER) in
action URLs, not the internal DB run ID (GITHUB_RUN_ID). This caused
"View job" links to return 404. Fixed in all 4 URL construction sites:
create-initial comment, update-comment-link, and both prompt generators.

Also pins Bun to 1.2.21 in CI — Bun 1.3.9 broke spyOn(fs, ...) which
failed 18 upstream image-downloader tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/forgejo/comment-logic.ts

@@ -34,6 +34,24 @@ function getServerUrl(): string {
   return process.env.GITHUB_SERVER_URL || "https://github.com";
 }
 
+// ---------------------------------------------------------------------------
+// Run index helper
+// ---------------------------------------------------------------------------
+
+/**
+ * Get the run identifier for Forgejo URLs.
+ *
+ * Forgejo uses the per-repo sequential run index in URLs (`/actions/runs/{index}`),
+ * not the internal database run ID. The Forgejo runner sets:
+ *   - GITHUB_RUN_ID     = internal DB primary key (e.g. 11943)
+ *   - GITHUB_RUN_NUMBER = per-repo sequential index (e.g. 661)
+ *
+ * GitHub uses GITHUB_RUN_ID in URLs, but Forgejo uses GITHUB_RUN_NUMBER.
+ */
+export function getForgejoRunIndex(): string {
+  return process.env.GITHUB_RUN_NUMBER || process.env.GITHUB_RUN_ID || "0";
+}
+
 // ---------------------------------------------------------------------------
 // Spinner
 // ---------------------------------------------------------------------------