Feature request: CLI flags/settings to hide default slash commands and agents from menu

Feature Request

Add the ability to hide/filter default slash commands and agents from the / menu via CLI flags or settings.json configuration.

Use Case

Power users with many custom commands/skills want to declutter the menu by hiding built-in commands they don't use. Currently:

• --tools and --disallowedTools control execution but not menu visibility
• disable-model-invocation: true prevents auto-invocation but items still appear in menu
• No settings.json option exists for menu visibility

Proposed Solutions

Any of these would work:

1. CLI flag: --hide-commands "help,clear,compact" or --show-commands "custom-only"
2. Settings.json: "hiddenCommands": ["/help", "/clear"] or "menuFilter": { "showBuiltins": false }
3. Per-scope control: Allow hiding at user/project/local scope levels

Additional Context

This would help users who:

• Have extensive custom skill libraries and want a cleaner menu
• Work in specific contexts where certain built-in commands are irrelevant
• Want to create focused, minimal interfaces for specific workflows