Daily improvement proposal 2026-07-25: Add README and docs link checks

[OpenCow42]

Summary

Add a lightweight check that verifies the README and documentation Markdown links still point to existing local files.

Why this is useful

The README now has a broad documentation index and doc/TESTING_AND_DEVELOPMENT.md keeps link verification as a manual docs-only step. A small script or CI check would catch stale local doc links before release notes, architecture pages, or testing guidance drift.

Proposed small change

• Add a small Markdown link check for repository-local links in README.md, CHANGELOG.md, and doc/*.md.
• Run it either as a documented script or as a lightweight GitHub Actions step alongside the existing macOS build/test workflow.

Evidence checked

• README.md documentation index links to many files under doc/ plus CHANGELOG.md.
• doc/TESTING_AND_DEVELOPMENT.md says docs-only changes should manually verify root README links point to existing files.
• Recent open issues cover known-folder namespacing and rate-limit backoff, not documentation link checking.
• Recent CI runs show the Xcode build/test workflow is active on pull requests and pushes.

Validation gate

Stitch should not work on this until Adrien explicitly approves this issue or asks Stitch to implement it.

Suggested acceptance check

• A documented command or CI step fails when a repository-local Markdown link in README.md or doc/*.md points to a missing file, and passes on the current valid documentation set.