session crash recovery and resumption

[yai-dev]

Summary

Add crash recovery and resumption for interrupted sessions.

Current State

The current runtime can persist session history, but an in-flight turn is not yet resumable after a process crash or interrupted request.

Proposed Design

1. Persist turn checkpoints around LLM/tool boundaries
2. Add an explicit resumption path in the current host/app layer
3. Reconcile pending tool calls safely when resuming
4. Define timeout/heartbeat semantics for marking a turn interrupted

Affected Areas

• packages/core/src/agent/agent-loop.ts
• session-store contracts in packages/core
• host/app route and session orchestration in packages/app

Acceptance Criteria

• Checkpoints are written at stable recovery points
• Interrupted turns can be resumed against a persistent session store
• Pending tool calls are replayed or skipped according to explicit semantics
• Manual resumption API is documented and tested
• Crash/recovery integration tests cover a mid-turn interruption path