feat(msg): add P2P messaging with self-hosted relay

Add `crab msg` command for direct messaging between coworkers using a
self-hosted Python3 HTTP relay. The relay runs on the host's machine and
can be exposed to the internet via cloudflared tunnel.

Commands:
- crab msg start [--expose]  Start relay (with optional tunnel)
- crab msg join <url>        Connect to a relay
- crab msg @name "text"      Send a message
- crab msg read              Check inbox
- crab msg listen            Live message stream
- crab msg stop              Stop relay

Zero new dependencies - relay uses Python3 stdlib, tunnel uses
cloudflared (optional). Bumps version to 0.9.0.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: MrFlounder