feat(web): replace the @ mention with an /agent command, and stage both switches until send (#122)

Co-authored-by: Claude Opus 5 (1M context) <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 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.
+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 `/agent` handoff does: picking a model stages it as a chip in the composer, and **sending** 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 the composer held (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).
 
@@ -47,9 +47,9 @@ There are four approval modes: `allow-all`, `deny-all`, `read-only` (only read-o
 ### Input and Shortcuts
 
 - Enter sends, Shift+Enter inserts a newline, and images can be pasted;
-- Typing `/` opens the slash menu: trigger context compaction (`/compact`) or toggle installed Skills — chosen Skills are sent along with the message in a `[use_skills]` block;
+- Typing `/` opens the slash menu: trigger context compaction (`/compact`), hand the conversation over to another Agent (`/agent`), switch the model (`/model`) — both switch commands appear in an active session only, since a draft has nothing to switch and picks its Agent and model up front — or toggle installed Skills — chosen Skills are sent along with the message in a `[use_skills]` block;
 - While a Task is running the input stays live and the toolbar keeps a single action button: an empty composer shows **Stop**, and typing turns it into **Send**, whose behavior follows the **mid-run send mode** from the toolbar's More-settings popover (a compact extensible settings panel, also available in draft state; the choice is remembered): **Steer** (default) delivers the text mid-run as a `[user_steering]` user message with the next turn, **Queue** holds the whole message server-side as a follow-up and auto-sends it as an ordinary new message when the run finishes (an "N queued" hint shows near the input until then; the queue survives page reloads);
-- Typing `@` mentions another Agent to hand the conversation over to it;
+- `/agent` and `/model` stage their pick instead of acting on it: the chosen Agent or model becomes a chip above the text body and nothing is sent yet, so you keep typing — Enter/Send is what hands the conversation over (a new chat for that Agent) or forks it onto the chosen model, carrying the text along; with an empty composer a default message is filled in, and the chip's × cancels. Both chips are cached with the draft, so they survive a reload or a trip to another conversation together with the text. A model fork additionally waits for the Session to be idle — it continues from the Session's Trace, which a running turn or a compaction is still writing — and a line above the composer says so while it waits;
 - When human approval is required, tool calls show inline allow/deny buttons in the message stream; the approval mode can be changed mid-Session;
 - While the engine waits out a reconnect backoff (≥2s), the retry line shows a live countdown to the next attempt with inline **Retry now** (skips the remaining wait) and **Give up** (the ordinary abort) controls;
 - When the model API rejects the Session's credentials (an authentication failure), the composer grays out and disables — recoverably: the Session pins only the model reference, and credentials come from the current Project config. The notice's primary button opens the Models page; saving a new API key there unlocks the composer by itself (open tabs unlock live via a `credentials_updated` event, and after a reload the composer stays unlocked because the credential update is newer than the recorded auth failure). A "Retry" button clears the state manually for another attempt (it re-arms if the key is still bad), a completed request always clears it, and "New Session" remains as the escape to a fresh draft. The disabled composer keeps its draft selectable, so a long message that failed to send can still be copied out.