Add auto-review target proof

[shiny-code-bot]

Summary

• add snapshot epoch/commit/head proof to auto-review targets with legacy JSON compatibility
• require proof-aware freshness for durable background duplicate reuse while keeping existing branch/head cleanup paths
• bump and refresh the background review target after acquiring the coordination lock so persisted runs match the reviewed snapshot
• cover legacy no-proof targets, epoch mismatches, duplicate rejection, app-server fixtures, and target collection

Verification

• cargo test -p codex-auto-review snapshot
• cargo test -p codex-core collect_target_records_current_snapshot_epoch
• cargo check -p codex-app-server
• cargo fmt --all --check
• git diff --check

Fixes #130