Validate fixed commit identity and parents for original review reads #632

Add a strict fixed-commit detail read for original review context. Require a full lowercase SHA-1, verify exact response identity and ordered nullable parent metadata, disable redirects and decode only after status validation. Add captured-account repository currency guards that discard obsolete results and typed failures. Preserve existing general branch/tag commit reads.

Use test-first API/repository coverage. No original coordinate synthesis or private write/UI is exposed; fresh MR membership, target availability and literal selection remain subsequent integration requirements. Related to MW-07 and #630.