feat(agent): model-agnostic AI agent bridge (PoC) + pin lets-hack demo to main

Add cmd_chat/agent: a headless client that joins a room via SRP, decrypts
broadcasts, and answers /ai <question> through a pluggable model provider
(ollama default + anthropic + openai-compatible + module:Class). Server and
zero-knowledge guarantees unchanged; the agent is just another encrypted client.

Also pin the lets-hack demo to a detached worktree of main (default) so running
it from dev still demos stable main without touching the working checkout.

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

cmd_chat/agent/__init__.py

@@ -0,0 +1,6 @@
+"""hack-house AI agent bridge — model-agnostic agents that join a room."""
+
+from .bridge import AgentBridge
+from .providers import Msg, Provider, make_provider
+
+__all__ = ["AgentBridge", "Msg", "Provider", "make_provider"]