stage-6: model selection and advanced commands

/model (show/change), /restart (owner), /logs, set_session_model API, model reset on /clear. 20 new tests (161 total).

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

src/claude_session.py

@@ -271,6 +271,20 @@ def get_active_session(channel_id: str) -> dict | None:
     return sessions.get(channel_id)
 
 
+def set_session_model(channel_id: str, model: str) -> bool:
+    """Update the model for a channel's active session. Returns True if session existed."""
+    if model not in VALID_MODELS:
+        raise ValueError(
+            f"Invalid model '{model}'. Must be one of: {', '.join(sorted(VALID_MODELS))}"
+        )
+    sessions = _load_sessions()
+    if channel_id not in sessions:
+        return False
+    sessions[channel_id]["model"] = model
+    _save_sessions(sessions)
+    return True
+
+
 def list_sessions() -> dict:
     """Return all channel→session mappings from active.json."""
     return _load_sessions()