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

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

packages/docs/content/sessions-and-traces.en.md

@@ -55,7 +55,7 @@ See `packages/core/src/trace/writer.ts` for the implementation.
 The head of a Trace (illustrative; one OmniMessage envelope per line):
 
 ```jsonl
-{"timestamp":"2026-07-18T03:10:22.531Z","type":"session_meta","payload":{"session_id":"session-2026-07-18-11-10-22-3f8a1c2d","provider":"deepseek","model_id":"deepseek-v4-pro","model_context_window":1000000,"system_prompt":"…","tools":[…],"thinking_level":"medium","agent_state":"/home/u/.penguin/data/default_project/agents/default_agent/agent_state","workspace":"/home/u/work"}}
+{"timestamp":"2026-07-18T03:10:22.531Z","type":"session_meta","payload":{"session_id":"session-2026-07-18-11-10-22-3f8a1c2d","provider":"deepseek","model_id":"deepseek-v4-pro","model_context_window":1000000,"system_prompt":"…","tools":[…],"agent_state":"/home/u/.penguin/data/default_project/agents/default_agent/agent_state","workspace":"/home/u/work"}}
 {"timestamp":"…","type":"event_msg","payload":{"type":"request_begin"}}
 {"timestamp":"…","type":"model_msg","payload":{"type":"text","role":"user","text":"Create hello.txt"}}
 {"timestamp":"…","type":"model_msg","payload":{"type":"tool_call","role":"assistant","name":"exec_command","arguments":"{\"cmd\":\"printf hi > hello.txt\"}","tool_call_id":"call_0"}}
@@ -79,6 +79,10 @@ Recovery requires that the Workspace and the model still exist. What recovery gu
 
 Special case: if the latest Trace file ends with a completed compaction, that context is closed as a whole — resume starts from an empty context; in summarize mode the `<context_summary>` is reconstructed and prepended to the first input after resume.
 
+## Model switch (/model)
+
+The Web's `/model` command changes models the way the @ handoff does: it creates a new Session under the same Agent via the ordinary session-creation API (the chosen model, **the source session's Workspace** — so files stay reachable), and the first message opens with a `<model_switch_from>` source block — the source session id, the absolute path of its latest Trace file, the Workspace, and the previous model pair — followed by whatever the user typed. The history is **not injected** into the new context: some models require thinking payloads and `fidelity` byte-for-byte when history is replayed, which cannot cross models — instead the model reads the source Trace file itself (JSONL, one message envelope per line) when it needs the earlier context. The source session and its Trace are untouched.
+
 ## Field fidelity
 
 Each content message's opaque provider `fidelity` payload (thinking signatures, phase labels, encrypted reasoning, …) is preserved verbatim in the Trace and sent back verbatim — some models require it byte-for-byte on history replay, and any rewriting would break compatibility. This is one reason the Trace stores raw OmniMessage envelopes rather than a post-processed format.