Surface context compaction lifecycle

[nikita-ashihmin]

Summary

• map context compaction start and completion items to an ACP tool-call lifecycle
• attach generic contextCompaction metadata so clients can provide specialized UI while older clients retain a standard other tool-call fallback
• replay completed context compactions consistently when loading session history

Verification

• npm run typecheck
• npm test (297 passed, 28 skipped)
• npm run build
• real Codex /compact run confirmed matching item/started and item/completed events with the same item ID