Feature: Skill subcommand autocomplete support

Feature Request

Skills that accept subcommands (e.g., /config-sync pull|push|status) have no way to surface those arguments in autocomplete. Currently, only the top-level skill name is autocompleted — any arguments the skill accepts are invisible to the user until they read the skill docs.

Current Behavior

• Typing /config- autocompletes to /config-sync
• After selecting /config-sync, there's no autocomplete for pull, push, status, etc.
• The only way to discover subcommands is to read the SKILL.md or invoke the skill without arguments

Proposed Solution

Add an optional subcommands field to the SKILL.md frontmatter:

---
name: config-sync
description: Sync config repo across machines
version: 1.0.0
subcommands:
  - name: pull
    description: Pull latest changes from remote
  - name: push
    description: Commit and push local changes
  - name: status
    description: Show sync status without making changes
---

After selecting /config-sync, the autocomplete system would then offer pull, push, status as completions.

Workaround

Currently the only workaround is to create separate skills for each subcommand (e.g., /config-sync-force instead of /config-sync force), which leads to skill sprawl and duplicated context in skill docs.

Impact

This would make the skill system more discoverable and reduce the need to split logical subcommands into separate skills just for autocomplete support.