Validate Markdown links and skill frontmatter in CI

[biggdawg320]

Closes #1.

Adds an offline validator for relative Markdown links in root README files, docs and skills, and YAML frontmatter in skills/SKILL.md files. Failures name the affected file and reason. Markdown parsing ignores code examples and supports reference links and destinations containing spaces/parentheses. External URLs and anchors are not fetched or validated.

Adds nine synthetic/current-content tests, development-only Markdown/YAML dependencies, a CI step, and contributor setup instructions. Collector runtime dependencies remain unchanged.

Validation:

• 9 focused validator tests pass; current repository documentation passes.
• Python compilation, whitespace check and detect-secrets scan pass.
• Full Windows unittest discovery: 38 pass, 2 fail. The same two failures occur on unchanged main: eBay persistence tests expect POSIX 0600 permissions, while Windows reports 0666. No unrelated permission changes included. Hosted Ubuntu CI remains to be verified.

This contribution was implemented and tested with Codex under Jim Hall's direction, with review of the parser behavior and regression cases.