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>

test/forgejo-comment-logic.test.ts

@@ -1,6 +1,7 @@
 import { describe, it, expect, beforeEach, afterEach } from "bun:test";
 import {
   FORGEJO_SPINNER_HTML,
+  getForgejoRunIndex,
   createForgejoJobRunLink,
   createForgejoBranchLink,
   createForgejoPrCompareUrl,
@@ -37,6 +38,46 @@ describe("FORGEJO_SPINNER_HTML", () => {
   });
 });
 
+// ---------------------------------------------------------------------------
+// getForgejoRunIndex
+// ---------------------------------------------------------------------------
+
+describe("getForgejoRunIndex", () => {
+  const origRunNumber = process.env.GITHUB_RUN_NUMBER;
+  const origRunId = process.env.GITHUB_RUN_ID;
+
+  afterEach(() => {
+    if (origRunNumber !== undefined) {
+      process.env.GITHUB_RUN_NUMBER = origRunNumber;
+    } else {
+      delete process.env.GITHUB_RUN_NUMBER;
+    }
+    if (origRunId !== undefined) {
+      process.env.GITHUB_RUN_ID = origRunId;
+    } else {
+      delete process.env.GITHUB_RUN_ID;
+    }
+  });
+
+  it("prefers GITHUB_RUN_NUMBER over GITHUB_RUN_ID", () => {
+    process.env.GITHUB_RUN_NUMBER = "661";
+    process.env.GITHUB_RUN_ID = "11943";
+    expect(getForgejoRunIndex()).toBe("661");
+  });
+
+  it("falls back to GITHUB_RUN_ID when GITHUB_RUN_NUMBER is not set", () => {
+    delete process.env.GITHUB_RUN_NUMBER;
+    process.env.GITHUB_RUN_ID = "11943";
+    expect(getForgejoRunIndex()).toBe("11943");
+  });
+
+  it("returns '0' when neither env var is set", () => {
+    delete process.env.GITHUB_RUN_NUMBER;
+    delete process.env.GITHUB_RUN_ID;
+    expect(getForgejoRunIndex()).toBe("0");
+  });
+});
+
 // ---------------------------------------------------------------------------
 // createForgejoJobRunLink
 // ---------------------------------------------------------------------------