fix: preserve YAML frontmatter in skill fill

[LorranHippolyte]

Problem

Running skill fill overwrites SKILL.md files with the raw LLM output, completely discarding the original YAML frontmatter (type, skillSlug, phases, etc.).
After filling, the MCP skill list returns 0 skills because skill discovery relies on the frontmatter to identify files.

Fixes #28

Root cause

skillFillService.ts was writing result.text directly to disk with no frontmatter handling:

// before
await fs.writeFile(skillPath, this.ensureTrailingNewline(result.text));

Fix

src/services/fill/skillFillService.ts

1. Extract the original frontmatter from the existing SKILL.md before calling the LLM
2. Strip any frontmatter from the LLM output (removeFrontMatter)
3. Reconstruct the file with the original metadata and status: filled

// after
const cleanBody = removeFrontMatter(result.text);
const updatedFrontMatter = originalFrontMatter
  ? { ...originalFrontMatter, status: 'filled' }
  : { status: 'filled' };
await fs.writeFile(skillPath, this.ensureTrailingNewline(addFrontMatter(cleanBody, updatedFrontMatter)));

All utilities (parseFrontMatter, removeFrontMatter, addFrontMatter) were already present in src/utils/frontMatter.ts — no new dependencies added.

Testing

21 unit tests passing. Pre-existing build failures related to tree-sitter type declarations are unrelated to this change.