Add Markdown linting

[rlippmann]

Summary

• added a low-noise .markdownlint-cli2.jsonc aligned with the context-compiler-example-integrations reference policy
• added ./scripts/validate_markdown.sh as the local Markdown lint command
• added a dedicated markdownlint CI job in .github/workflows/ci.yml
• mechanically cleaned up existing repository-owned Markdown only where required to satisfy the lint baseline

Notes

• establishes a clean Markdown lint baseline without changing drafting or runtime behavior
• existing Markdown was mechanically cleaned up only as required for lint compliance
• local lint command: ./scripts/validate_markdown.sh
• dedicated CI job: markdownlint
• validation completed:
  • ./scripts/validate_markdown.sh — passed
  • uv run pre-commit run --all-files — passed
  • uv run pytest — passed
• no runtime behavior changes
• no package-version changes