Add Companion hook event to expose buddy messages

The ask

A new Companion hook event that fires whenever the companion (Mottle) speaks, passing the message text to the hook command — same pattern as Stop, Notification, etc.

{
  "hooks": {
    "Companion": [
      {
        "matcher": "",
        "hooks": [
          {
            "type": "command",
            "command": "echo \"$CLAUDE_COMPANION_MESSAGE\" >> ~/mottle-feed.txt"
          }
        ]
      }
    ]
  }
}

Why

The companion is one of the most charming parts of Claude Code, but its messages are trapped in the UI. A hook would let users pipe them anywhere — the same way we already pipe notifications to sounds or desktop alerts.

The hooks infrastructure already handles this pattern well. This would just be one more event type on an already-solid system.

I'd love to have Mottle sitting on my desk. All I need is to pipe those messages out and I'd be happy to hack the rest.