feat: autofocus prop for the embedded chat composer

[camielvs]

Description

Adds an autoFocus prop to the embedded chat so a host can land the user in the composer on load. The motivating flow is tangle-ui's new-project page (stack leading up to TangleML/tangle-ui#2762): it opens the tangent page, creates a session, and wants the cursor already in the box so the user can type immediately.

The composer's textarea is disabled until the socket joins and the agent roster arrives, so a plain DOM autofocus would land on a disabled field and do nothing. Instead the prop waits for the composer to accept input, then focuses once per mounted session — no refocus on reconnect, on a run finishing, or after the user clicks elsewhere in the host page. Focus uses preventScroll: true so the host page never jumps.

Threaded through all three layers of the contract:

• <Chat autoFocus> in @tangent/embed-react, assigned as an element property before sessionId so the first render already knows.
• autoFocus property + auto-focus attribute on <tangent-chat> for raw-element hosts.
• ComposerShell owns the focus effect; ChatInput / SubagentTabView pass it through, so it also works when agentId targets a sub-agent thread rather than being silently ignored.

No server change was needed for the "start a session, say nothing" half of the flow — newSession("", bundleId) already queues an empty prompt that is never sent.

Screenshots

N/A — no visual change; the only difference is where focus sits on load.

How to test

1. pnpm dev
2. In a host app (or a scratch page) pointing at this instance, create a session without an opening prompt and render the chat with the prop:

   const { sessionId } = await newSession("", "tangle-oss", { name: "New project" });
   <Chat sessionId={sessionId} autoFocus />

3. On load, the cursor lands in the composer as soon as it becomes enabled — typing goes straight into it, and the host page does not scroll.
4. Click elsewhere in the host page, then let the agent finish a run or drop/restore the socket: focus stays where the user put it.
5. Without autoFocus, behaviour is unchanged (nothing takes focus).

Notes

• Prop-contract change, so tangle-ui needs a published @tangent/embed-react to get the typed prop (currently 0.1.1 — happy to bump in this PR or a follow-up). Not strictly required: a host can set element.autoFocus = true / the auto-focus attribute and pick up the behaviour from the runtime bundle alone.
• Verification is pnpm validate + build:embed + the wrapper's .d.ts. This repo has no embed harness page (only public/bundle-ui-harness, which is for bundle UI), so the focus path has not been exercised in a browser here — it will be on the tangle-ui side.
• Deliberately no imperative focusComposer() method: refocusing after mount is more annoying than useful. Easy to add later if a host surface needs it.

🤖 Generated with Claude Code