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>

docs/usage.md

@@ -81,6 +81,7 @@ jobs:
 | `path_to_bun_executable`         | Optional path to a custom Bun executable. Skips automatic Bun installation. Useful for Nix, custom containers, or specialized environments                                             | No       | ""            |
 | `plugin_marketplaces`            | Newline-separated list of Claude Code plugin marketplace Git URLs to install from (e.g., see example in workflow above). Marketplaces are added before plugin installation             | No       | ""            |
 | `plugins`                        | Newline-separated list of Claude Code plugin names to install (e.g., see example in workflow above). Plugins are installed before Claude Code execution                                | No       | ""            |
+| `platform`                       | Platform to run on: `auto` (default, auto-detects), `github`, or `forgejo`. See [Forgejo Setup](./forgejo.md)                                                                          | No       | `auto`        |
 
 ### Deprecated Inputs