Feature: Allow custom skill ordering/pinning in slash command menu

Problem

When working on a project with multiple custom skills (.claude/skills/), the slash command menu lists them in discovery/alphabetical order with no way to control positioning. Frequently-used skills get buried among less common ones.

Proposed Solution

Add a way to control skill ordering in the slash command menu. Some options:

1. priority field in SKILL.md frontmatter — numeric value (lower = higher in list):

``yaml
---
description: My most-used skill
priority: 1
---
``

1. pinned: true field — pin skills to the top of the menu:

``yaml
---
description: My most-used skill
pinned: true
---
``

1. Ordering config in settings.json — centralized control:

``json
{
"skillOrder": ["implement-ticket", "safe-commit", "security-review"]
}
``

Current Workaround

Prefix directory names with numbers (1-implement-ticket/, 2-safe-commit/), which makes the slash commands ugly.

Use Case

Projects with 4+ custom skills where 1-2 are used far more frequently than others. Being able to pin those to the top reduces friction when invoking them via /.