feat(prd-1): implement M1 - platform detection and action.yml modifications

- Add src/platform/detector.ts with 3-tier detection (explicit input,
  FORGEJO env var, GITHUB_SERVER_URL heuristic)
- Add 'platform' input to action.yml (auto/github/forgejo)
- Add 'Detect Platform' shell step as first composite action step
- Split bun installation: oven-sh/setup-bun for GitHub, curl fallback
  for Forgejo runners
- Condition 'Revoke app token' step to skip on Forgejo
- Pass resolved PLATFORM env var to run step
- Add 14 unit tests covering all detection paths

Progress: M1 complete (1/9 milestones) - M2 and M5 now unblocked

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

prds/1-forgejo-platform-support.md

@@ -228,7 +228,7 @@ The upstream repo has 13 workflows in `.github/workflows/`. Strategy:
 
 ### Phase 1 - Foundation (parallel, no dependencies)
 
-- [ ] **M1: Platform detection + `action.yml` modifications** — Add `platform` input to `action.yml`. Create `src/platform/detector.ts` with explicit input + env-var auto-detection. Add conditional logic to skip "Revoke app token" step on Forgejo. Fix bun installation to handle Forgejo runners (direct curl fallback).
+- [x] **M1: Platform detection + `action.yml` modifications** — Add `platform` input to `action.yml`. Create `src/platform/detector.ts` with explicit input + env-var auto-detection. Add conditional logic to skip "Revoke app token" step on Forgejo. Fix bun installation to handle Forgejo runners (direct curl fallback).
   - Files: `action.yml` (MODIFIED), `src/platform/detector.ts` (NEW)
   - Success: `detectPlatform()` returns "forgejo" in Forgejo runner, "github" in GitHub runner. `action.yml` doesn't error on Forgejo.