feat: add --session-id option to timmy chat CLI

Allows specifying a named session for conversation persistence.
Use cases:
- Autonomous loops can have their own session (e.g. --session-id loop)
- Multiple users/agents can maintain separate conversations
- Testing different conversation threads without polluting the default

Precedence: --session-id > --new > default 'cli' session

src/timmy/cli.py

@@ -128,14 +128,25 @@ def chat(
         "-n",
         help="Start a fresh conversation (ignore prior context)",
     ),
+    session_id: str | None = typer.Option(
+        None,
+        "--session-id",
+        help="Use a specific session ID for this conversation",
+    ),
 ):
     """Send a message to Timmy.
 
-    Conversation history persists across invocations. Use --new to start fresh.
+    Conversation history persists across invocations. Use --new to start fresh,
+    or --session-id to use a specific session.
     """
     import uuid
 
-    session_id = str(uuid.uuid4()) if new_session else _CLI_SESSION_ID
+    if session_id is not None:
+        pass  # use the provided value
+    elif new_session:
+        session_id = str(uuid.uuid4())
+    else:
+        session_id = _CLI_SESSION_ID
     timmy = create_timmy(backend=backend, model_size=model_size)
 
     # Use agent.run() so we can intercept paused runs for tool confirmation.