feat(merge_requests): reconfirm original commit context

[sjungwon03]

Summary

Combine strict original MR commit reads through one captured account. Fetch target commit detail and literal diff only after selected membership is confirmed, then reconfirm MR origin and membership before returning. Changed origins/membership and contradictory known parents fail; obsolete results and typed errors are discarded. Unknown source/parent metadata and literal omission flags remain intact.

Closes #634

Validation

• dart format .: 969 files; zero outstanding changes.
• flutter analyze: no issues.
• All five package suites: 7,821 tests.
• 51 new adapter-based tests fail against the unimplemented reader and pass after implementation.
• Generated files: not applicable; named record aggregate of existing DTOs.
• Device validation: not applicable to this repository-only slice; no live GitLab validation claimed.

Checklist

• Targets dev; linked issue; Conventional Commit and DCO sign-off.
• Test-first behavior and updated contract/parity documentation.
• English repository prose; dummy test credentials only.
• Layout/themes, dependencies and localization: unchanged.

Notes for reviewers

Sequential reads are not an atomic snapshot or write-safety proof. Original parent/reference selection, complete literal hunk/coordinate validation and guarded controller/private-save reservation/recovery UI remain follow-ups. Captured guards do not cancel dispatched reads or authentication recovery.

[sjungwon03-ai]

Reviewed commit 06608e0: All four PR blobs match the reviewed local commit. One captured client performs target-project fixed commit and literal diff reads only after membership validation, then refreshes MR identity/origin and exhausts membership again. Unknown parent/source metadata remains unknown; both-known ordered parents must agree. All six phases discard obsolete success and typed failures; current failures remain typed. Full format/analyze and 7,821 local tests pass, including 51 new test-first cases. No blocking findings. Sequential reads are not atomic, and original refs/coordinates or private-write eligibility are not inferred; guarded literal selection and controller/private-save UI remain follow-ups.