Add frontmatter completeness checker

[ai-hustle-bro]

Closes #53.

Summary

• Add a frontmatter-completeness drift checker for context/*.md and patterns/*.md files.
• Warn when name, description, or last_updated is missing or blank.
• Register the checker in runDriftCheck and include it in verbose checker counts.
• Normalize user-visible paths to POSIX-style separators in a few scanner/checker outputs so the existing test suite passes on Windows.

Validation

• npm run typecheck
• npm test
• npm run build
• git diff --check

AI assistance

Implemented with AI assistance and manually verified via the validation commands above.

[ai-hustle-bro]

Synced this PR with current main and added a small test-only guard for Windows environments that cannot create file symlinks without Developer Mode/elevated rights.

Validation rerun locally on Windows / Node 24:

npm run typecheck
npm test
npm run build
git diff --check

Results:

• npm run typecheck: passed
• npm test: 14 test files passed; 185 passed / 1 skipped. The skipped case is the symlinked-bin test when Windows returns EPERM for symlink creation.
• npm run build: passed
• git diff --check: passed

Current head: a12f49b.