[Good First Issue] Support markdown frontmatter in local files

[mverab]

When optimizing local markdown files with /geo:optimize, preserve frontmatter (YAML/TOML) instead of treating it as content.

Example:

Input:

---
title: My Page
description: My description
---

# Content here

Expected: Frontmatter preserved, only "Content here" optimized.

What's needed:

1. Detect frontmatter in markdown files
2. Parse and preserve it separately
3. Only optimize the content body
4. Reassemble with original frontmatter

Resources:

• Python frontmatter library: https://pypi.org/project/frontmatter/
• Current file handling in commands