Feature Request: Support argument completions/autocomplete in custom skills

Feature Request

Problem

Custom skills can define an argument-hint to show expected arguments, but there's no way to provide actual autocomplete suggestions when users type /skill-name .

Use Case

I have a /ag skill that loads agents into the session. Valid arguments include:

• Team shortcuts: it, marketing, ops, pm, finance, legal
• Agent names: devops-engineer, sql-pro, python-pro, etc.

Currently users must memorize or reference documentation for valid options.

Proposed Solution

Add an argument-completions field to skill frontmatter:

---
name: ag
description: Load agents into session
argument-hint: "team | agent-name"
argument-completions:
  - it
  - marketing
  - ops
  - devops-engineer
  - sql-pro
---

When user types /ag , these would appear as autocomplete suggestions.

Alternatives Considered

• VSCode snippets (requires separate setup)
• Pre-command hooks (complex, external tooling)
• Documentation (poor UX)

Additional Context

This would significantly improve discoverability for skills with many valid argument options.