Support Codex and Claude commands in the existing chat UI

[llmreseacher]

For Codex and Claude Code, the slash picker exposes skills and MonoCode shortcuts without a way to choose the agent's commands. For example, /status should show useful status for the current graphical conversation.

Proposed behavior:

• Add All / Agent commands / Skills filters and /agent:name to resolve collisions with skills.
• Implement GUI counterparts for commands that MonoCode can support through the current session's controls and provider adapters.
• Keep the transcript, composer, draft and attachments in the existing chat; do not hand off to a terminal or another provider session.
• Label commands without a GUI implementation as unsupported, keep their draft on invocation, and avoid forwarding them as ordinary model prompts.

PR #98 implements status/context, model and permissions controls, fast/effort settings where available, compaction, planning, diff, stop and help. Full CLI command parity remains outside this implementation.

[prkl78]

/remote-control (/rc) is also unsupported. right now it's sent to the model as a plain prompt.

cause (meat proxy): MonoCode spawns Claude Code in SDK mode (--input-format stream-json --output-format stream-json, buildClaudeSpawnArgs, src/lib/harness/claudeProtocol.ts:224). Built-in REPL commands don't exist in that transport, and the Claude harness registers no commands provider, so isNativeCommandPrompt() (src/lib/skills.ts:149) returns false and the text falls through.

Two asks:

1. Short term: mark unknown /-commands as unsupported
2. Longer term: Remote Control needs an interactive session or claude remote-control server mode. Upstream headless request: Feature Request: claude remote-control --headless — daemonizable remote control without TTY dependency anthropics/claude-code#30447.