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

## 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 — #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.

Start with acp.rs, where available_commands_update is currently logged, and trace how the existing @-mention autocomplete is implemented in the composer. Persist each agent’s latest command name and description, feed the relevant commands into a fuzzy-filterable slash dropdown, and verify insertion, filtering, dismissal, mention scoping, and refresh behavior.

Written by the indexing model from the issue text.