Studio chat completion: slash command and skill menu (/) #2164

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

Outcome

Typing / at the start of a message in an existing chat opens a menu of the session's slash commands and skills, and choosing one inserts a command chip.

Scope

• Add the / suggestion menu, allowed only at the start of the message in an existing session, using the per-session slash-command projection. Skills appear as /<skill-name> and are labeled as skills so users can tell them from file-backed commands.
• Show clear loading, empty, error, and unsupported states; hide the feature when the slashCommands capability is off.
• Insert an atomic chip that sends as /name. An unknown command typed by hand still passes through unchanged, as the daemon already does.

Acceptance

• The menu lists the commands and skills the session reports and filters as you type.
• It does not open mid-message or in a new chat that has no session yet; selection never sends the prompt.
• Component tests and task studio:check pass.

Delivery

Bounded UI change over the existing projection.

Depends on: #2161, #2162.

Fully or partially written by an AI agent.