chore(ci): add GitHub Actions workflow for PR checks

[mgratzer]

Parent: #1

Summary

Add a CI workflow that runs markdown linting and cross-skill consistency checks on every pull request.

Problem / Motivation

Without CI, lint violations and cross-skill inconsistencies (mismatched pipeline references, divergent commit conventions) can slip through review.

Proposed Solution

Create .github/workflows/ci.yml that runs on pull requests.

Implementation Details

1. Markdown linting step: npx markdownlint-cli2 "**/*.md"
2. Cross-skill consistency checks (from docs/testing.md):
   • Verify all skills reference the same pipeline order
   • Verify commit format convention is consistent across skills
3. Keep it simple: Single job, no matrix, just npx + grep checks

Alternatives Considered

• Pre-commit hooks — Local-only, not enforceable. CI is the right place for shared checks.
• Mega-Linter — Full linting suite. Overkill for a markdown-only repo.

Acceptance Criteria

• .github/workflows/ci.yml created
• Workflow triggers on pull requests to main
• Markdown linting runs and fails on violations
• Cross-skill consistency checks run
• docs/ci.md created documenting the workflow
• CLAUDE.md docs table updated to include ci.md