feat: Add slashcommand api

toju-app/src/app/domains/chat/README.md

@@ -144,6 +144,10 @@ The chat domain consumes the custom emoji picker from `domains/custom-emoji`. Me
 
 Custom image emoji are stored locally through `DatabaseService` and sync peer-to-peer with `custom-emoji-summary`, `custom-emoji-request`, `custom-emoji-full`, and `custom-emoji-chunk` data-channel events. Uploads use the same image types as profile avatars (`.webp`, `.gif`, `.jpg`, `.jpeg`) and are capped at 1 MB. The composer inserts saved custom emoji as readable inline aliases such as `:party:`, so they can sit in the middle of text like `This is :party: cool`; sending rewrites known aliases to stable `:emoji[id](name)` tokens and proactively pushes the referenced assets to connected peers alongside the outgoing message, edit, or reaction. Rendering resolves stable tokens against synced known assets and shows a sized placeholder image until the asset arrives; deferred markdown placeholders use readable `:name:` aliases instead of raw tokens. A repair request is still sent if a token is seen without a local asset. Seen remote emoji do not enter the picker automatically; right-click a custom emoji in chat or on a custom emoji reaction and choose **Add to emoji library** from the context menu. Right-click a saved custom emoji inside the picker to remove it from the local library. The full picker includes search that filters Unicode emoji by common terms and saved custom emoji by name.
 
+### Slash commands
+
+The composer renders a Discord-style autocomplete menu when the user types `/`. Results merge first-party built-in commands with plugin-registered commands (`PluginUiRegistryService.slashCommandRecords`), filtered by surface (`commandSurface` input: `server` exposes global + server commands, `direct` exposes only global) and by query. Built-in commands live in `chat-builtin-slash-commands.rules.ts`; each defines fixed `text` that is sent as a normal chat message through the composer's `messageSubmitted` output. The default built-in is `/lenny`, which posts `( ͡° ͜ʖ ͡°)`. Plugin commands run their own `run` callback instead. Picking a command with no options runs it immediately; a command with options pre-fills `/name ` for argument entry. Slash input is intercepted and never posted verbatim; `/text` that matches no command falls through as a normal message. See the plugins domain README for the `api.commands` registration contract.
+
 ## Domain rules
 
 | Function | Purpose |