feat(web,server,core): attach files to a message from the composer (#121)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

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

@@ -47,6 +47,7 @@ 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;
+- The "+" menu holds the input add-ons: **image upload**, **file attachment** and goal mode. An attachment can be any type (up to 20 at a time, ≤ 10MB each and 12MB in total; an oversize pick is refused before it is read, so nothing is uploaded to earn the rejection); selected files show as removable chips above the text body in the order they were picked, and a message with attachments and no text is sendable. On send the files are written into the Session's scratchpad — deleted with the Session — and the message gains an `[attached file: <path>]` line per file, which the conversation renders as an "Attached files" notice: the bytes never enter the conversation, the model opens each file by path with its ordinary file tools;
 - 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);
 - `/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;