perf: reuse fresh specialist research artifacts

[DivyamTalwar]

Summary

• add a seven-day, content-addressed cache for reusable specialist JSON artifacts
• invalidate on normalized company, research depth, model, full prompt, or agent-template changes
• restore only regular JSON specialist files; never cache logs, verifier output, or synthesizer output
• stage writes before replacement and document the privacy/lifecycle boundary

Verification

• Rust CI runs cargo clippy --all-targets --all-features -- -D warnings
• Rust CI runs cargo test --all-targets --all-features
• unit tests cover material-input invalidation, TTL expiry, and exclusion of verifier/log artifacts

This optimization preserves the trust model: cached facts are inputs only; verifier and final synthesis always rerun.