feat(desktop): autocomplete ACP slash commands

[kevinmanase]

Summary

• forward ACP available_commands_update notifications as encrypted semantic observer snapshots
• persist the latest bounded command catalog per owner and agent, including trusted archive hydration
• add grouped slash-command autocomplete at message start or after leading agent mentions
• route start-of-message selections through the chosen agent mention and register its exact pubkey

Behavior

Typing / at the start of a message shows commands advertised by agent members in the channel. Typing / after one or more leading @Agent mentions narrows suggestions to those agents. Duplicate command names stay grouped by provider, empty updates remove stale commands, and catalogs survive app restarts.

Testing

• pnpm --dir desktop test (3,466 passing)
• pnpm --dir desktop check
• pnpm --dir desktop typecheck
• pnpm --dir desktop build
• cargo test -p buzz-acp available_commands_update_emits_complete_semantic_snapshot
• cargo clippy -p buzz-acp --all-targets -- -D warnings
• cargo fmt --all -- --check

Fixes #2528

[rshiozaki]

We'd like to see this land, so we did a full review of this PR against today's main (22cdda4) and revived the branch. TL;DR: the implementation is solid, and we've prepared a conflict-free, test-green rebase that @kevinmanase or maintainers are welcome to take over: https://github.com/rshiozaki/buzz/tree/rebase/acp-slash-autocomplete

Review findings

Strengths:

• Persistence design is right. available_commands_captured observer frames feed a per-owner+agent localStorage catalog; both the live and archive ingestion paths record it, and restart survival is covered by tests.
• Scoping avoids the union-menu trap. With no mention typed, suggestions group per agent rather than merging into one ambiguous list; selecting inserts @Agent /cmd and registers the exact pubkey; leading mentions narrow the list.
• Advertised data is treated as untrusted: name ≤128 chars (no whitespace), description ≤512, ≤256 commands per agent, deduped, re-sanitized on hydrate, plain-text rendering only.
• Staleness handling: newer (ts, seq) wins, and an empty available_commands_update is honored as an authoritative removal (tested).

Gaps worth follow-ups (not blockers in our view):

• No visual distinction between built-in commands and discovered skills — the catalog is a flat {name, description}.
• A command that has gone stale by send time fails silently; there's no send-time revalidation or freshness hint in the UI.
• Sanitization bounds lengths/counts but doesn't strip control or bidi characters.
• One undisclosed limitation worth stating in the PR body: catalogs only populate for agents the current user owns (observer frames are encrypted to the owner), so a teammate's agent shows no commands. Reasonable for v1, but the longer-term shape probably wants a signed per-agent advertisement event any channel member can read — happy to write that up as a design issue if there's interest.
• With two or more leading mentions, an inserted /cmd addresses all mentioned agents rather than one.

What the rebased branch adds

All feature credit to @kevinmanase; we started from the conflict resolutions in #3537 (thanks to its author too) and brought it the rest of the way:

• Rebased onto today's main — the original branch was 687 commits behind and merge-dirty; three conflict sites resolved (MessageComposer.tsx composer-dock changes, localStorageQuota.ts prefixes, check-file-sizes.mjs).
• File-size gate satisfied without an override. The feature pushed MessageComposer.tsx to 1,047 lines; the original commit slipped under the 1,000-line gate via an override mechanism that has since been removed from the repo. We extracted the shared autocomplete insertion path and picker keydown chain into a sibling hook (useComposerAutocompleteInsertions.ts), following the existing composer hook pattern — the composer is now 968 lines, below where main already is (999).
• Green on today's main: full desktop unit suite (5,578 tests, 0 failures), tsc --noEmit, biome, and the agent-harness Rust tests including this feature's snapshot test.

Related work, for cross-reference: this implements #2528 and also covers the ask in #5741; #6567 builds the picker-chip UI for the same space but without persistence or the typed-/ path.

Happy to open the rebased branch as a fresh PR, or hand it off however is easiest — whichever the maintainers prefer.