feat(gc): reclaim stale cache key schemas

[jleni]

Closes #750

Summary

• record the cache-key recipe version in entry metadata and the SQLite index
• add explicit kache gc --stale-schema reclamation for older and unknown legacy entries
• preserve schema provenance through imports and index rebuilds while leaving ordinary GC and upgrades non-destructive
• reuse the GC lock, recent-access grace, and durable-upload pins during the sweep

Validation

• cargo test --workspace --all-features
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• focused store migration, import, reclamation, and CLI tests
• cargo fmt --all -- --check
• git diff --check