Claim PRIME session lock. This makes the current session the exclusive recipient of:
- Incoming Signal messages (via signal-bridge)
- TTS playback rights
- Any other inbound comms routing

Steps:
1. Call `whoami_info` to get your session ID.
2. Write that session ID to `~/.local/state/opencode/prime.lock` (create parent dirs if needed).
3. Confirm: "PRIME locked to session {id}"

This always overwrites — no contention check, no confirmation needed. The current session becomes PRIME immediately.