feat(rn): [P1] Slash command sheet

[liuyidi]

Summary

Mobile slash palette: typing / opens ActionSheet. Built-ins: /new /stop /clear /compact /model /help. Dynamic skill commands after Discover Skills lands.

WebUI reference

• webui/src/components/thread/ComposerPalettes.tsx
• webui/src/lib/chat/slashSkills.ts
• webui/src/components/thread/ThreadComposer.tsx

RN landing

• Wire in FloatingChatComposer.tsx (+ optional SlashCommandSheet.tsx)
• /stop → existing abort; /new → new session; /model → open model picker

Acceptance

• / opens sheet with built-ins
• /stop / /new / /model behave as above
• Dynamic skill commands when Skills catalog available

Depends

• feat(rn): [P0] Composer model picker (for /model)
• feat(rn): [P1] Discover: live Skills + MCP templates (for dynamic skill cmds)

Follow-up (not this issue)

@ mentions (CLI/MCP) — see lib/chat/mentions.ts, MentionTokens.tsx, send mcp_presets.