feat: add bounded history and tool-event retrieval

[wanghuacan]

Summary

• add bounded, provenance-safe tool-call and tool-result retrieval
• add composable history windows with actual-message neighbors and centered anchors
• preserve complete future user source while exposing truncation, completeness, and index freshness metadata
• update the Distill skill to guide flexible history reconstruction
• retain Python 3.9 compatibility and serialize index refreshes

Verification

• history-window contract: 82 checks passed
• non-frontend regression suite: 685 checks passed
• Python 3.9 session import check passed
• monitored Codex CLI end-to-end Distill journey passed for current and legacy message history
• stale tool-event locator was safely rejected when refresh was unavailable