Add broken-image pre-commit hook + CI check

[jmxpearson]

Summary

Phases 1, 2, and 4 (PR #32) added pre-commit hooks for image compression, a size cap, and code quality. There is no hook that verifies image references resolve. The dibs-web01 rot (separate issue) is exactly the failure mode this would catch.

Proposal

A small Python script run as a local hook in .pre-commit-config.yaml:

• For each staged *.html and *.md, scan for <img src="..."> and ![alt](url) references.
• For relative paths: verify the target exists in the repo.
• For external URLs: HEAD with a short timeout; fail on 4xx/5xx (with the same conservative accept list as lychee).
• Skip data URIs and template Liquid expressions like {{ site.url }}/....

Mirror in CI so contributors who skip the local install still get caught.

Acceptance

• scripts/check_image_refs.py (stdlib + minimal deps)
• Wired into .pre-commit-config.yaml
• Added to .github/workflows/pre-commit.yml and/or site-health.yml
• README documents what it does and how to override (config file analogous to .image-size-overrides)

Pairs with

The dibs-web01 migration issue. This hook is the prevention; that one is the cure.