Require active agent participants for channel agent mode

[rogerchappel]

Summary

• Require shared-channel agent mode and realtime voice sessions to target an active agent participant: on_call or proactive.
• Keep mention_only agents available only for explicit mentions/wake phrases, not ongoing two-way agent mode.
• Treat direct agent DMs as active agent conversations even when their stored participation mode is mention_only.
• Split chat UI channel agents into mentionable vs active participants and route plain channel messages to a single active agent participant when present.

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: changes shared-channel agent routing semantics and agent-mode availability. Scope is limited to channel invocation guards and chat-page routing behavior.