feat(desktop): slash-command autocomplete in composer from ACP available_commands_update (follow-up to #919)

[zacklavin11]

Problem

Agents already advertise their slash commands over ACP, but Buzz Desktop users have no way to discover them — typing / in the composer shows nothing, so you have to memorize and type full command names (/ce-commit-push-pr, /ffrc-slide-forge, …) by hand. For users whose agents load large skill libraries (hundreds of Claude Code skills/plugins), this makes the shipped slash pass-through hard to use in practice.

Background — plumbing already exists

#919 (merged 2026-06-09) shipped slash-command pass-through to ACP connectors and explicitly deferred this piece:

No UI discovery/autocomplete yet — available_commands_update is logged; persisting + surfacing it to the desktop composer is a follow-up.

So today acp.rs receives each agent's command list via the ACP available_commands_update session update and drops it after logging. This issue tracks the follow-up: persist that list and feed it to a composer autocomplete.

Proposed UX

• Typing / at the start of the message — or after a leading @AgentName mention — opens a fuzzy-filterable dropdown, styled like the existing @-mention autocomplete.
• The list shows the commands the mentioned agent last advertised (name + one-line description from available_commands_update). With no mention yet, fall back to the union of commands from agents in the channel, grouped by agent.
• Enter/Tab inserts the command; typing continues to filter. Escape dismisses and leaves the literal / text.
• Commands persist per agent (last-received available_commands_update), so the list is available even before the agent's session is warm; refresh on each new update.

Non-goals

• No command execution semantics in Buzz — feat(acp): pass slash commands through to ACP connectors #919's contract stands: connectors own parsing and execution; Desktop only helps compose the text.
• No new protocol work — the ACP update already carries name + description.

Prior art

• Existing @-mention autocomplete in the composer (interaction pattern to reuse).
• Zed's ACP client surfaces available_commands_update as composer completions — same protocol, same shape.