Slash commands and skills gain menuDescription, policyGate, and custom argument autocompletion

What

• Commands and skills can now define a getArgumentCompletions callback; the prompt input calls this while typing to generate custom suggestions for a command's arguments.
• Command/skill definitions gained a menuDescription field: a shorter description shown in menu/listing UI, separate from the full description.
• Command/skill definitions gained a policyGate field: a policy object with a .policy check that can hide or disable a command.
• Internally, policyGate is now set up as a lazy getter during registration (rather than a plain assigned property), and getArgumentCompletions is passed through registration alongside it.

Why These fields give command and skill authors more control over how their commands appear and behave in menus: a concise label for tight UI space, a way to restrict visibility based on policy, and live, tailored argument suggestions instead of generic ones.