Implement Zed client support for the draft [ACP Session Notices
RFD](https://agentclientprotocol.com/rfds/session-notices), allowing
agents to surface operational advisories without turning them into
conversation history.

- Advertise the v1 notices capability behind the existing ACP beta
feature flag
- Keep notices in live, session-local state, with independent local
dismissal and no persistence, transcript/export entries, protocol
acknowledgement, or changes to generation status.
- Render dismissible callouts above the composer in a bounded scrolling
area. Support info, warning, and error treatments, with a generic
presentation for unknown severities and plain-text titles/descriptions.
- Provide screen-reader labels, keyboard dismissal with focus
restoration, and a single scrolling surface for long descriptions.
- Add a shared notice component and designer previews for severity
variants, title-only/detailed content, narrow layouts, and overflowing
stacks.

Open **workspace: open component preview**, then select **Agent →
Session Notice**.

<img width="829" height="316" alt="image"
src="https://github.com/user-attachments/assets/2a6eba57-465e-41e8-b99f-5f57c00199d8"
/>


Release Notes:

- N/A