* test: add unit tests for skill frontmatter validation
Cover _validate_skill_frontmatter logic:
- Valid minimal and full-field skills
- Missing SKILL.md, missing frontmatter, invalid YAML
- Required field validation (name, description)
- Unexpected key rejection
- Name format: hyphen-case, no leading/trailing/consecutive hyphens
- Name and description length limits
- Angle bracket rejection in description
* test: fix unused variables flagged by ruff F841
Replace unused tuple elements with _ and add assertions on
msg/name return values in success-path tests.
* test: address review feedback on unused variables
* test: consolidate validation tests into single module
Move the UTF-8/windows-locale test from test_skills_router.py into
test_skills_validation.py and remove test_skills_router.py to eliminate
duplicated assertions and future maintenance drift.
* fix: match assertion strings to actual validation messages
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
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