feat(commits): validate fixed review head and parent reads

[sjungwon03]

Summary

Add a strict fixed-commit detail read that confirms the exact requested SHA and validates ordered nullable parent metadata before exposing it to future original review context. Add captured-account repository currency guards that discard obsolete results and typed failures. Existing general branch/tag detail reads retain their contract.

Closes #632

Validation

• dart format .
• flutter analyze reports no warnings
• Full flutter test in all five packages
• Test first: 54 API and 18 repository cases; the strict API/repository behavior failed with UnimplementedError before implementation. General branch read remains covered.
• Manual app/device validation: no UI is added; live private-instance/device verification is not claimed.

Checklist

• Targets dev, links one issue, Conventional Commit and DCO sign-off
• Updated contract documentation and MW-07 status
• English source/tests/docs; dummy credentials and example hosts
• No dependencies added or models changed; no generated files edited

Notes for reviewers

The strict API requires a full lowercase SHA-1 and exact returned identity; reported parents must be canonical, distinct and not self-referential. Absent/null remains unknown; empty means reported root; server order is preserved. It requests stats=false, checks status before plain JSON parsing and disables redirects while retaining captured read-only OAuth recovery. OriginalCommit reads through one captured account/target; caller rechecks currency after await. No parent selection, blank/empty-tree reference synthesis, membership proof, original coordinate or private-write/UI eligibility follows. Those checks and guarded review UI remain follow-ups; MW-07/image/file drafts remain in progress. See docs/fixed-commit-review-reads.md.

[sjungwon03-ai]

Reviewed commit f237a5d: Exact fixed-head route/identity, immutable ordered nullable parents, malformed/self/duplicate parent rejection, status-first sanitized decoding and redirect options are correct. Captured read-only authentication and originalCommit currency guards preserve current typed failures and discard obsolete results. Existing branch/tag detail compatibility remains covered. All six PR blobs match the reviewed local commit. Full format/analyze and 7,770 local tests pass, including 72 new test-first/compatibility cases. No blocking findings. No parent choice, original references/coordinates, MR membership or private-write eligibility is inferred; guarded integration and UI remain follow-ups.