fix: improve error message for marketplace plugin commands

- Detect namespaced commands (containing ':') from Claude marketplace plugins
- Provide clear error message explaining marketplace plugins are not supported
- Point users to .claude/commands/ as alternative for custom commands
- Fixes issue where /daplug:run-prompt gave ambiguous 'command not found'

Closes #1682

src/hooks/auto-slash-command/executor.ts

@@ -202,7 +202,7 @@ export async function executeSlashCommand(parsed: ParsedSlashCommand, options?:
   if (!command) {
     return {
       success: false,
-      error: `Command "/${parsed.command}" not found. Use the slashcommand tool to list available commands.`,
+      error: parsed.command.includes(":") ? `Marketplace plugin commands like "/${parsed.command}" are not supported. Use .claude/commands/ for custom commands.` : `Command "/${parsed.command}" not found. Use the slashcommand tool to list available commands.`,
     }
   }