Expose compact Auto Review ledger

[shiny-code-bot]

Summary

• Add a bounded Auto Review ledger projection from the durable run store.
• Inject the ledger as a request-only developer message only when active or recent actionable review state exists.
• Classify the ledger as its own context source so /context shows Auto Review prompt overhead.
• Avoid creating Auto Review store directories for idle repos during normal turns.
• Tighten actionability so inactive in-flight runs and clean cancellations do not tax every prompt.

Validation

• cargo test -p code-core review_store
• cargo test -p code-core auto_review_ledger_developer_message_has_distinct_context_source
• ./build-fast.sh
• git diff --check

Agent Review Notes

• Agents reviewed correctness and long-term dogfood/token-efficiency goals.
• Addressed feedback by moving ledger reads to once per turn, using non-mutating open_existing, adding in-flight activity gates, omitting clean cancellations, and softening lazy-detail wording until Add lazy Auto Review detail retrieval #328 lands.

Closes #327