fix(context): retain sent thread messages in fallback

[yanpgwang]

Summary

• project agent.thread_message_sent into event-ledger model context as assistant-authored output
• preserve destination Thread and Agent identity in the same structured envelope used for received cross-Thread messages
• cover both the pure projection and PrepareTurn fallback when the private provider transcript is unavailable

Why

The public primary and child Thread ledgers already durably record outgoing tasks, follow-ups, and reports as agent.thread_message_sent. The fallback context projection ignored those events, so a coordinator could forget that it had delegated work and a child could forget its prior report whenever the lossless provider transcript could not be used.

The normal provider-transcript path is unchanged; this repairs only safe legacy/recovery reconstruction from the official event ledger.

Verification

• make verify
• focused fallback and Thread-message projection regression tests

Reference

• https://platform.claude.com/docs/en/managed-agents/multiagent-orchestration