Add markdown lint enforcement via CI and repo pre-commit hook

[Copilot]

This change introduces repository-level markdown lint enforcement in both CI and local commit flow. It adds a single lint policy source and applies it consistently across pull requests, pushes to main, and staged markdown commits.

• CI enforcement

  • Adds .github/workflows/markdownlint.yml to run markdownlint-cli2 on pull_request and push (main).
  • Sets explicit workflow token scope (contents: read).

• Local commit guard

  • Adds .githooks/pre-commit to lint staged *.md files before commit.
  • Filters out skills/**/templates/** paths to mirror repo lint exclusions.
  • Treats missing npx as a soft skip with warning (non-blocking).

• Shared lint policy

  • Adds .markdownlint-cli2.yaml as the canonical rule config and path scope.
  • Documents rationale for disabled rules to keep policy intentional and maintainable.

• Repository docs/context alignment

  • Updates README.md, STRUCTURE.md, and CLAUDE.md to reflect:
    • new markdown CI workflow
    • repo hook setup (git config core.hooksPath .githooks)
    • markdown lint now part of the repo’s static quality gate

# .github/workflows/markdownlint.yml
jobs:
  markdownlint:
    runs-on: ubuntu-latest
    permissions:
      contents: read
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: 22
      - run: npx --yes markdownlint-cli2