feat(prd-1): complete M9 - documentation & upstream merge validation

- Created docs/forgejo.md: comprehensive setup guide (prerequisites,
  quick start, platform detection, GitHub differences, CI workflows,
  upstream merge process, known limitations, troubleshooting)
- Updated README.md with Forgejo support mention and docs link
- Updated CLAUDE.md with Forgejo architecture section
- Added platform input to docs/usage.md inputs table
- Added 5 Forgejo FAQ entries to docs/faq.md
- Upstream merge validated: zero conflicts (based on latest upstream)
- PRD #1 100% complete: all milestones M1-M9 done, 794 tests passing

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

prds/1-forgejo-platform-support.md

@@ -291,9 +291,11 @@ The upstream repo has 13 workflows in `.github/workflows/`. Strategy:
   - Depends on: M7
   - Success: Agent mode works on Forgejo. Self-test workflow passes. CI tests pass. Note: tag mode (issue trigger, PR review) not yet E2E tested — code paths are wired but await real-world validation.
 
-- [ ] **M9: Documentation & upstream merge validation** — Document Forgejo-specific setup (runner configuration, token setup, `platform: forgejo` input). Create `CLAUDE.md` for the project. Perform a fresh `git fetch upstream && git merge upstream/main` to verify conflicts are minimal and easily resolvable. Document the merge process.
+- [x] **M9: Documentation & upstream merge validation** — Created comprehensive `docs/forgejo.md` setup guide covering prerequisites, quick start (tag + agent mode), platform detection, GitHub vs Forgejo differences (auth, API, bun install, comment URLs), CI workflows, upstream merge process with conflict mitigation, known limitations, and troubleshooting. Updated `README.md` (Forgejo in description, feature bullet, docs link), `CLAUDE.md` (Forgejo architecture section with key files and integration points), `docs/usage.md` (`platform` input in table), `docs/faq.md` (5 Forgejo FAQ entries). Upstream merge validation: `git merge upstream/main` → zero conflicts — branch based on latest upstream `ade221f`. All 794 tests pass.
+
+  - Files: `docs/forgejo.md` (NEW), `README.md` (MODIFIED), `CLAUDE.md` (MODIFIED), `docs/usage.md` (MODIFIED), `docs/faq.md` (MODIFIED)
   - Depends on: M8
-  - Success: Clean or trivially-resolvable upstream merge. Clear setup docs.
+  - Success: Zero-conflict upstream merge. Complete Forgejo setup documentation with troubleshooting guide.
 
 ### Milestone Dependency Graph