Add a frontmatter-completeness drift checker

[theDakshJaitly]

Context

Add a checker that warns when a context/*.md or patterns/*.md file is missing recommended frontmatter fields: name, description, last_updated.

Scope

• For each context/*.md and patterns/*.md file, parse frontmatter and warn on any missing recommended field.
• Warning-level severity.

Where

• New checker: src/drift/checkers/frontmatter-completeness.ts.
• Reuse the existing parseFrontmatter helper in src/drift/frontmatter.ts (the edges checker already uses it — see how checkEdges is wired in the per-file loop in src/drift/index.ts).
• Register in src/drift/index.ts; tests in test/checkers.test.ts.

Acceptance criteria

• Warns when name / description / last_updated is missing.
• Clean when all recommended fields are present.
• Registered + counted in src/drift/index.ts.
• npm run typecheck && npm test && npm run build pass.

New to mex? Good first issue — reuses parseFrontmatter and follows the existing checker pattern. See CONTRIBUTING.md.