Add Markdown linting

[rlippmann]

What changed

• added a low-noise .markdownlint-cli2.jsonc based on 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 needed to satisfy the lint baseline
• excluded .tmp-uv-cache from linting because it is transient generated cache content

Why

• establish a clean, low-noise Markdown lint baseline for repository-owned documentation
• keep Markdown validation separate from runtime, test, and conformance validation
• align the repository with the current Context Compiler cross-repository Markdown linting standard

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)

Validation completed:

• ./scripts/validate_markdown.sh — passed
• uv run pre-commit run --all-files — passed
• uv run pytest — passed

Additional notes:

• existing Markdown was mechanically cleaned up only as required for lint compliance
• no runtime behavior changes
• no package-version changes