[FEATURE] Auto-completion for slash command arguments

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

custom skills (slash commands) often accept a fixed set of argument values — for example: /workflow dev /workflow groom /workflow qa_reopen.
the argument-hint frontmatter field only shows descriptive text during tab completion.
users must remember valid arguments and type them manually, which slows discovery and invites typos.

Proposed Solution

add new frontmatter field (e.g., argument-completions) in SKILL.md that defines completable argument values:

  ---
  name: workflow
  argument-hint: <type> <ticket-or-url>
  argument-completions:
    - dev
    - groom
    - qa-reopen
    - code-review
  ---

typing /workflow + tab would either:

• show these as selectable options in a dropdown, similar to how slash commands themselves are completed today.
• toggle through the accepted argument-completions (i prefer this behavior)

Alternative Solutions

• relying on argument-hint text to document valid values - works but requires some memorization
• adding validation inside the skill body to reject invalid args - catches errors but doesn't help with discovery

Priority

Low - Nice to have

Feature Category

Interactive mode (TUI)

Use Case Example

I've built a YAML-driven workflow engine exposed as a single /workflow skill with multiple workflow types (dev, groom, qa_reopen, code_review).
new team members will need to discover which types exist. argument completions would make this self-documenting.

Additional Context

this applies broadly to any skill with a finite set of valid first arguments (e.g., /deploy staging|production, /db migrate|rollback|seed) and so on...