fix(tui): preserved autocomplete for no-arg slash prompts

- Propagated builtin allowArgs metadata into TUI autocomplete entries.

- Let no-arg slash-looking prompts fall through to prompt-composer completions while keeping argument-capable commands scoped.

- Added regressions for /settings @ and /settings #copy.

packages/coding-agent/src/modes/prompt-action-autocomplete.ts

@@ -95,11 +95,13 @@ function getPromptActionPrefix(textBeforeCursor: string): string | null {
 }
 
 export class PromptActionAutocompleteProvider implements AutocompleteProvider {
+	#commands: SlashCommand[];
 	#baseProvider: CombinedAutocompleteProvider;
 	#actions: PromptActionDefinition[];
 	#basePath: string;
 
 	constructor(commands: SlashCommand[], basePath: string, actions: PromptActionDefinition[]) {
+		this.#commands = commands;
 		this.#baseProvider = new CombinedAutocompleteProvider(commands, basePath);
 		this.#basePath = basePath;
 		this.#actions = actions;
@@ -118,8 +120,13 @@ export class PromptActionAutocompleteProvider implements AutocompleteProvider {
 			leadingSlashStart !== null && !hasPromptTextBeforeCursorLine
 				? textBeforeCursor.slice(leadingSlashStart)
 				: null;
-		if (commandText?.includes(" ")) {
-			return this.#baseProvider.getSuggestions(lines, cursorLine, cursorCol);
+		const spaceIndex = commandText?.indexOf(" ") ?? -1;
+		if (commandText !== null && spaceIndex !== -1) {
+			const commandName = commandText.slice(1, spaceIndex);
+			const command = this.#commands.find(cmd => cmd.name === commandName || cmd.aliases?.includes(commandName));
+			if (command && (!("allowArgs" in command) || command.allowArgs !== false)) {
+				return this.#baseProvider.getSuggestions(lines, cursorLine, cursorCol);
+			}
 		}
 
 		const promptActionPrefix = getPromptActionPrefix(textBeforeCursor);

packages/coding-agent/src/slash-commands/builtin-registry.ts

@@ -2437,6 +2437,7 @@ export const BUILTIN_SLASH_COMMAND_DEFS: ReadonlyArray<BuiltinSlashCommand> = BU
 	command => ({
 		name: command.name,
 		aliases: command.aliases,
+		allowArgs: command.allowArgs === true,
 		description: command.description,
 		subcommands: command.subcommands,
 		inlineHint: command.inlineHint,

packages/coding-agent/src/slash-commands/types.ts

@@ -16,6 +16,8 @@ export interface BuiltinSlashCommand {
 	name: string;
 	aliases?: string[];
 	description: string;
+	/** Whether the command consumes text after the command name. */
+	allowArgs?: boolean;
 	/** Subcommands for dropdown completion (e.g. /mcp add, /mcp list). */
 	subcommands?: SubcommandDef[];
 	/** Static inline hint when command takes a simple argument (no subcommands). */