Studio chat: slash-command and agent-mention completion #2160

Parent: #1736. Not part of the 2026-09-24 design baseline batch tracked in #1779.
Blocked by: #1853 (sequencing).

Context

Studio's chat composer is plain text. It offers no discovery for slash commands, for skills (every discovered skill is also available as /<skill-name>), or for agent mentions, although the daemon already supports all three and reports them through the slashCommands and agents capabilities.

This was not tracked anywhere we could find: not in #1736's sub-issues, not in #1847 through #1851 (#1851 covers MCP inventory insertion only), and not in #1853, which covers the skills inventory and review. A design already exists as an open prototype PR, stacklok/mecatl-prototypes#39 ("slash-command discovery and agent mentions"): capability-gated BFF routes, a TipTap-based composer with autocomplete menus and atomic chips, side-thread support, and help rows. This issue brings it into Studio.

Outcome

A chat user can discover and insert slash commands (including skills) and agent mentions from the composer, in main chats and side threads, only where the connected deployment supports them. Prompts stay plain text on the wire.

Scope

Delivered through the sub-issues below, in this order. Each is its own bounded change.

• BFF contract for agent mentions and per-session slash commands (Studio chat completion: BFF contract for agent mentions and slash commands #2161)
• Composer editor foundation with plain-text preservation and atomic chips (Studio chat completion: composer editor foundation with plain-text chips #2162)
• Agent mention completion (@) (Studio chat completion: agent mention menu (@) #2163)
• Slash command completion (/), including skills (Studio chat completion: slash command and skill menu (/) #2164)
• Side-thread composer parity (Studio chat completion: side-thread composer parity #2165)
• Capability-aware guidance, help rows, and user docs (Studio chat completion: help rows, guidance, and user docs #2166)

Acceptance

• Each sub-issue meets its own acceptance and task studio:check passes.
• With the capabilities off, the composer behaves exactly as it does today and the help page says why the feature is unavailable.
• Sending a draft with chips produces the same plain-text prompt the daemon would receive from typing it.

Delivery

Bounded sub-issues, each an independent PR to main. Use the published SDK only and keep the browser behind the BFF. The editor library is a new web dependency; the editor-foundation sub-issue records that decision against #1779's rule of no external UI-kit dependency.

Fully or partially written by an AI agent.