Fix channel agent mode membership leakage

[rogerchappel]

Summary

• Reject channel-scoped realtime agent mode sessions when the selected agent is not an eligible channel member.
• Disable channel agent mode in the chat UI unless the selected agent can speak in the active channel.
• Carry channel scope through thread sends and realtime voice session requests.

Verification

• pnpm test -- 'src/app/api/runtimes/[id]/talk/realtime/session/route.test.ts' src/lib/realtime-voice-client.test.ts src/app/api/chat/route.test.ts
• pnpm typecheck
• git diff --check origin/main
• Pre-push hook: pnpm typecheck && pnpm build

Risk

Medium: affects channel-scoped agent invocation and realtime voice session routing. Changes are limited to membership checks, chat UI gating, and request scope propagation.