docs: add AGENTS.md and document issue-driven workflow

- Add AGENTS.md as the cross-agent entry point (lean; points to
  .gooserules and docs/AI_* for detail)
- Document the Forgejo issue-driven workflow (report → triage →
  implement → close) and API patterns in .gooserules
- Add Forgejo step to the pre-change checklist
- Gitignore .forgejo-token

.gitignore

@@ -1,6 +1,7 @@
 .env
 .env.local
 .env.*.local
+.forgejo-token
 node_modules/
 dist/
 target/