feat(btmsg): add group agent messenger CLI

Python CLI tool for hierarchical multi-agent communication.
SQLite-backed (WAL mode), agent identity via BTMSG_AGENT_ID env var.

Features:
- inbox/read/send/reply — message CRUD with read tracking
- contacts — role-based communication hierarchy enforcement
- history — per-agent conversation view
- status — all agents with tier/role/model/unread counts
- register/allow — agent and contact management
- notify — single-line notification for agent injection
- Short ID prefix matching for convenience

Also: change default Claude model to opus-4-6

v2/src/lib/providers/claude.ts

@@ -16,5 +16,5 @@ export const CLAUDE_PROVIDER: ProviderMeta = {
     supportsResume: true,
   },
   sidecarRunner: 'claude-runner.mjs',
-  defaultModel: 'claude-sonnet-4-20250514',
+  defaultModel: 'claude-opus-4-6',
 };