Fix auto-review store migration awareness

[shiny-code-bot]

Summary

• preserve visibility of pre-scoped auto-review runs from CODEX_LAB_HOME/auto-review/runs/*.json after the scoped store migration
• keep new scoped runs.json / outputs/*.json data authoritative over duplicate legacy run ids
• avoid auto-review awareness git/diff probing unless there is loadable legacy data, scoped store data, or a live pending/running review
• make legacy store reads best-effort so corrupt retired artifacts cannot hide valid scoped store data

Refs #126.

Validation

• cargo fmt --manifest-path codex-rs/Cargo.toml --all --check
• git diff --check
• cargo test -p codex-auto-review
• cargo test -p codex-core auto_review_awareness
• read-only agent review passes on migration compatibility, hot-path awareness detection, and legacy/new precedence