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/entrypoints/update-comment-link.ts

@@ -10,6 +10,7 @@ import {
 import {
   updateForgejoCommentBody,
   createForgejoPrCompareUrl,
+  getForgejoRunIndex,
 } from "../forgejo/comment-logic";
 import { detectPlatform } from "../platform/detector";
 import {
@@ -54,7 +55,9 @@ export async function updateCommentLink(
   const platform = detectPlatform();
 
   const serverUrl = GITHUB_SERVER_URL;
-  const jobUrl = `${serverUrl}/${owner}/${repo}/actions/runs/${process.env.GITHUB_RUN_ID}`;
+  const runIdentifier =
+    platform === "forgejo" ? getForgejoRunIndex() : process.env.GITHUB_RUN_ID;
+  const jobUrl = `${serverUrl}/${owner}/${repo}/actions/runs/${runIdentifier}`;
 
   let comment;
   let isPRReviewComment = false;