feat: restore prompt-cache hit-rate diagnostics

[cbusillo]

Summary

• restore prompt-cache hit-rate display in /status when cached input tokens are non-zero
• add a configurable footer status-line item: prompt-cache-hit-rate
• document Audit token and prompt-cache diagnostics parity #401 token diagnostics parity state and narrow release-note wording to avoid telemetry-present overclaims

Validation

• cargo test -p codex-tui prompt_cache
• INSTA_UPDATE=always cargo test -p codex-tui status_snapshot
• ./build-fast.sh

Review

• Focused read-only review agent found stale snapshots and release-note overclaim; both fixed before this PR.
• Background auto-review is still failing before findings because the repository-wide diff scope exceeds its configured size limit.