Add automated SKILL.md linting and format standardization

While SKILL.md files are generally well-structured, there are minor formatting inconsistencies across skills:

- Heading levels vary (some use \`##\`, others use \`###\` for similar sections)
- List formatting varies (some use \`-\`, others use \`*\`)
- Code block indentation is inconsistent
- Frontmatter field ordering varies

## Proposal

1. Add \`markdownlint\` configuration for automated format checking
2. Standardize heading levels across all SKILL.md files
3. Ensure consistent use of \`-\` for bullet lists
4. Document formatting conventions in \`CONTRIBUTING.md\`

This improves polish and helps AI agents parse files consistently.