feat(prd-1): implement M4a - Forgejo REST data fetching (PR/issue basics)

- Add src/forgejo/data/fetcher.ts: REST-based data fetcher replacing
  GraphQL queries with parallel Octokit REST calls (PR details + files
  + commits via Promise.all, issue details, file status mapping, SHA
  computation, TOCTOU title/body protection, user display name)
- Add test/forgejo-data-fetcher.test.ts with 31 test cases covering
  field mapping, parallel fetch, null handling, and edge cases
- Output matches FetchDataResult type for downstream compatibility
- Comments/reviews left empty (M4b scope)

Progress: 33% complete (M1, M2, M4a done) - next: M4b or M5

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

prds/1-forgejo-platform-support.md

@@ -243,7 +243,7 @@ The upstream repo has 13 workflows in `.github/workflows/`. Strategy:
 
 ### Phase 2 - Core Data Fetching (the hard part)
 
-- [ ] **M4a: Forgejo REST data fetching — PR/Issue basics** — Implement REST-based fetching for PR details, issue details, changed files, and commits. Produce output matching the `FetchDataResult` type that existing code expects.
+- [x] **M4a: Forgejo REST data fetching — PR/Issue basics** — Implement REST-based fetching for PR details, issue details, changed files, and commits. Produce output matching the `FetchDataResult` type that existing code expects.
   - Files: `src/forgejo/data/fetcher.ts` (NEW)
   - Depends on: M1, M2
   - Success: Can fetch PR/issue title, body, author, state, files, commits from Forgejo REST API.