feat(prd-1): implement M3 - Forgejo CI workflows

- ci.yml: 3-job CI (test, prettier, typecheck) on grunt runner
- test-action.yml: agent-mode self-test (weekly + PR path triggers)
- upstream-sync.yml: weekly upstream check with labeled issue dedup
- Updated PRD checkboxes for M3 completion

Progress: 78% complete (7/9 milestones) - M8 (e2e testing) next

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

prds/1-forgejo-platform-support.md

@@ -245,7 +245,8 @@ The upstream repo has 13 workflows in `.github/workflows/`. Strategy:
   - Depends on: M1
   - Success: Token is available without OIDC on Forgejo. GitHub path unchanged.
 
-- [ ] **M3: Forgejo CI workflows** — Create `.forgejo/workflows/` with CI, self-test, and upstream sync workflows. Document runner configuration requirements.
+- [x] **M3: Forgejo CI workflows** — Created `.forgejo/workflows/` with three workflows adapted for Forgejo `grunt` runner: `ci.yml` (3 separate jobs: test, prettier, typecheck using curl-based bun install + `xternal/checkout`), `test-action.yml` (agent-mode self-test with `FORGEJO_PLATFORM_TEST_OK` marker, runs weekly + on PRs to src/action.yml/.forgejo), `upstream-sync.yml` (weekly upstream check, creates/updates issue with `upstream-sync` label including commit log, diff stat, and merge instructions).
+
   - Files: `.forgejo/workflows/ci.yml`, `.forgejo/workflows/test-action.yml`, `.forgejo/workflows/upstream-sync.yml` (all NEW)
   - Success: CI runs tests on push. Upstream `.github/workflows/` don't interfere.