Studio chat completion: composer editor foundation with plain-text chips #2162

Parent: #2160. Design reference: stacklok/mecatl-prototypes#39 (open prototype).

Outcome

The chat composer can host inline completion and atomic chips while every existing composer behavior (drafts, attachments, queued messages, Enter to send, IME composition) keeps working and the sent prompt stays plain text.

Scope

• Replace the plain textarea with a rich-text editor that serializes to the same plain-text prompt the daemon receives today. The prototype uses TipTap with StarterKit, Placeholder, Mention, and Suggestion.
• Record the dependency decision: the editor library is a new web dependency, and Studio design review and parity tracking #1779 forbids an external UI-kit dependency. Approve or choose another approach in this issue before merging.
• Add an atomic chip node whose text form is the typed form (@name, /name), with no completion menus yet.
• Keep the local shared controls as the visual foundation.

Acceptance

• With no completion configured, typing, pasting, drafts, attachments, queueing, and sending behave as before; existing composer tests pass unchanged.
• A prompt containing chips round-trips to the same plain text.
• IME composition does not send the prompt.
• Component tests and task studio:check pass.

Delivery

Bounded; no behavior change visible to users beyond the editor surface.

Depends on: #1847.

Fully or partially written by an AI agent.