Persist realtime voice transcripts in chat

[rogerchappel]

Summary

• Adds a realtime transcript callback to VoiceAgent
• Persists final realtime user and assistant transcript events through /api/chat/messages
• Updates the visible chat/thread state immediately, then swaps in the persisted message id when available

Why

The OpenClaw realtime relay audio path works, but final transcript events were only logged as voice diagnostics. That made the audio exchange work while the CrewCMD message thread stayed empty.

Local verification

• pnpm test -- src/lib/gateway-client-realtime.test.ts src/lib/realtime-voice-client.test.ts src/lib/runtime-capabilities.test.ts
• pnpm typecheck
• git diff --check origin/main..HEAD
• pre-push: pnpm typecheck && pnpm build

Manual test notes

With realtime voice active, speak to the agent and confirm:

• /talk/realtime/session returns 200
• /talk/realtime/relay continues returning 200
• final user transcript appears in the CrewCMD thread
• final assistant transcript appears in the CrewCMD thread
• audio exchange still continues