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

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

packages/docs/content/web-app.en.md

@@ -33,7 +33,7 @@ The interface language (中文 / English / system) and theme (light / dark / sys
 
 ### Creating a Conversation
 
-A new conversation starts as a draft: pick the Agent, the Workspace (via a server-side directory browser), the approval mode, the model, and the thinking level before sending the first message. The Session is created on first send, and from then on its model and Workspace are locked. Switching the thinking level or the model makes the switched-to value the new default: the level is written back to the selected Agent's `model.thinking_level` immediately, and the picked model carries over as the next conversation's default; a running session keeps the level and model it was created with (the input area shows them read-only).
+A new conversation starts as a draft: pick the Agent, the Workspace (via a server-side directory browser), the approval mode, the model, and the thinking level before sending the first message. The Session is created on first send, and from then on its model and Workspace are locked. Switching the thinking level or the model in the draft makes the switched-to value the new default: the level is written back to the selected Agent's `model.thinking_level` immediately, and the picked model carries over as the next conversation's default. Inside an active session the thinking level is a per-turn parameter: the composer's picker starts out showing the Agent config's level and auto-follows it until touched (sends omit the level, so config edits keep taking effect); a pick sticks for the session and rides on every subsequent send (never written back to the Agent config); the model stays locked per session — the `/model` command switches models the way the @ handoff does: it opens a new session for the same Agent on the picked model, keeping the current Workspace, whose first message carries a `<model_switch_from>` source block (source session id, Trace file path, previous model) followed by whatever was left in the composer (an interface-language auto-line when empty). In the new session that block collapses into a "switched model" banner linking back to the source conversation, and the model reads the source Trace file itself when it needs the earlier history.
 
 There are four approval modes: `allow-all`, `deny-all`, `read-only` (only read-only tools pass), and `always-ask`. See [Tools and Approvals](/tools).