fix(analysis): harden --commit option edge cases and add tests

- Fix resolveOutputFile to preserve empty string (stdout mode)
- Inline tryRevParse into resolveCommitHash preserving exception cause
- Normalize blank --commit values as null to avoid silent failures
- Preserve original input subdirectory scope in worktree mode
- Add CommitAnalysisContext and CommonAnalyserParameters unit tests
- Document --commit/--local-changes mutual exclusivity in parser docs

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