Add Codex and Claude chat commands with command and skill filters

[llmreseacher]

What changed

Codex and Claude Code slash commands now have GUI counterparts in the current MonoCode chat. /status shows the session's model, permissions, project and context above the composer, while the transcript and input stay visible. The picker adds remembered All / Agent commands / Skills filters and /agent:name for explicit command selection when a skill has the same name.

Supported actions cover status/context, model selection, permissions, model-advertised fast/effort settings, compaction, planning, diff, stop and help. They use the current session's existing callbacks. /cli:name is a compatibility alias with the same chat behavior.

Closes #97.

Why

The existing Codex/Claude slash picker exposes skills and MonoCode shortcuts without a way to select agent commands. Users need command controls that retain the graphical conversation and act on that conversation's settings and history.

This is not full CLI command parity. Catalog entries without a chat implementation are labeled Not supported in chat; invoking one shows an error and preserves the draft. Commands do not spawn a terminal, fork provider history, or turn unsupported command text into a model prompt. /plan <request> intentionally submits a planning turn, and /compact uses provider-owned compaction. Status displays MonoCode's current session data; account rate limits remain in the existing usage footer.

UI

/status in an existing conversation, with a pending attachment preserved (real SessionPane and composer in the mocked browser fixture):

See supported commands and behavior. Run npm run dev and open /tests/agent-commands.html for the browser fixture.

Validation

• npm run check passed on macOS: 125 Vitest files / 1,293 tests, TypeScript, Rust formatting, Clippy with warnings denied, and 205 Rust tests.
• 13 browser regression checks passed, covering both providers, unchanged transcript/composer identity, session callbacks, autocomplete and filters, unsupported-command drafts, busy-state behavior, attachments, planning and empty chats. IPC is mocked; no real agents or model requests run in the fixture.
• macOS arm64 Tauri application build passed. Linux and Windows have not been exercised locally.

Checklist

• I ran npm run check
• This PR is focused on command access for the existing Codex and Claude Code chat integrations
• I did not mix unrelated changes