Fix OpenClaw realtime Talk session compatibility

[rogerchappel]

Summary

• Translate CrewCMD realtime relay calls to OpenClaw's unified talk.session.* API for OpenClaw 2026.5.18+
• Subscribe realtime relay SSE streams to the current talk.event channel while retaining the legacy event subscription
• Update runtime capability metadata to advertise the unified Talk session/event methods

Why

OpenClaw 2026.5.18 replaced the old talk.realtime.* RPC surface with talk.session.*, talk.client.*, and talk.event. CrewCMD was still calling talk.realtime.session, which caused realtime session startup to return 502 even though the STT/TTS fallback still worked.

Local verification

• pnpm test -- src/lib/gateway-client-realtime.test.ts 'src/app/api/runtimes/[id]/talk/realtime/session/route.test.ts' 'src/app/api/runtimes/[id]/talk/realtime/relay/route.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

For the walk test, run with:

NEXT_PUBLIC_CREWCMD_REALTIME_VOICE=1 pnpm dev

Expected realtime path in logs:

• POST /api/runtimes/<id>/talk/realtime/session 200
• event stream connects to /talk/realtime/events
• no immediate fallback POST /api/stt unless realtime session startup fails

Fallback remains available: if realtime session startup fails, the existing recorded STT -> chat -> TTS path should still work.