Add lazy Auto Review detail retrieval

[cbusillo]

Summary

• add a read-only auto_review_detail model tool for bounded Auto Review run/finding detail lookup by stable run id and optional finding id
• project durable sidecar output into capped metadata-rich responses with explicit missing-run, missing-sidecar, invalid-sidecar, invalid-finding, and missing-finding errors
• keep the compact Auto Review ledger lightweight while pointing agents to the lazy detail tool

Fixes #328

Validation

• cargo test --manifest-path code-rs/Cargo.toml -p code-core detail_lookup
• cargo test --manifest-path code-rs/Cargo.toml -p code-core auto_review_detail_tool_requires_only_run_id
• cargo test --manifest-path code-rs/Cargo.toml -p code-core compact_ledger
• cargo test --manifest-path code-rs/Cargo.toml -p code-core open_existing_read_only_does_not_create_outputs_dir
• ./build-fast.sh

Agent review

• planning agents converged on a focused core tool + store projection implementation
• initial review agents found one medium issue: the tool used a mutable store opener; fixed by adding open_existing_read_only
• final review agents reported no blocking/high/medium issues