fix(prd-7): M3 security review — self-trigger guard, actor permissions, trigger bypass, platform validation

- Add self-mention/bot loop prevention in actor.ts (blocks actor matching bot_name)
- Switch Forgejo permission check to actor-level via getCollaboratorPermissionLevel
  with fallback to token-level repos.get() when collaborator API returns 403
- Close empty-body PR review comment trigger bypass (now returns false instead of true)
- Add platform detection cross-validation (reject PLATFORM=forgejo on github.com)
- Document URL builder defense-in-depth rationale in comment-logic.ts
- Update/add tests across 6 test files (actor, permissions, trigger, platform, mcp, m7)

Progress: 50% complete (3/6 milestones) — next: M4 test coverage review

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

test/install-mcp-server.test.ts

@@ -328,6 +328,8 @@ describe("prepareMcpConfig", () => {
       savedServerUrl = process.env.GITHUB_SERVER_URL;
       // Set platform to forgejo for all tests in this block
       process.env.PLATFORM = "forgejo";
+      // Ensure cross-validation doesn't reject PLATFORM=forgejo
+      process.env.GITHUB_SERVER_URL = "https://git.example.com";
     });
 
     afterEach(() => {