[Feature Request] Autocomplete skill/slash-command arguments from argument-hint

Bug Description
Feature request: autocomplete for skill/slash-command arguments based on argument-hint.

Right now argument-hint looks like display-only text: it shows greyed out after the command name, but nothing completes from it. It would be much more useful if Claude Code could autocomplete arguments from it.

Proposal, in two levels:

1. Enumerated values: when argument-hint lists choices like "[sync|backup|diagnose]", Tab cycles through them and filters as you type.
2. Dynamic candidates: an optional frontmatter field (e.g. argument-completions) that points to a script or file glob whose output becomes the completion list. Examples: branch names for a worktree skill, file IDs for a content skill, profile names for an export skill.

Why: skills with subcommands or known argument sets are common, and today you have to remember the exact spelling or open SKILL.md to check. Level 1 needs no new frontmatter and would cover most cases.

Environment Info

• Platform: linux
• Terminal: zed
• Version: 2.1.294
• Feedback ID: 7b60a0eb-6bf2-4ebb-bc1f-16be90bec0c0