Handoff-style /model switch command and per-turn thinking level (#64)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/server/test/errors.test.ts

@@ -360,7 +360,6 @@ describe("stream-error-watcher (LLM / Environment errors)", () => {
         model_context_window: 100000,
         system_prompt: "",
         tools: [],
-        thinking_level: "default",
         agent_state: agentState,
         workspace: "/tmp/w",
       }),