Preserve realtime voice consult context across restarts

[rogerchappel]

Summary

• keep recent final realtime voice turns in a per-session continuity buffer
• append that voice context to the next OpenClaw realtime consult args
• cover the context merge and session-scoped buffer with focused relay tests

Verification

• pnpm test -- src/lib/realtime-voice-gateway-relay.test.ts
• pnpm typecheck
• git diff --check
• pre-push: pnpm typecheck && pnpm build

Risk

Medium: this changes the context sent to OpenClaw realtime consult tool calls, but only by adding recent final voice turns for the same session.