Fix link-check workflow config, pin and harden actions, prune dead README links

[Copilot]

The link-check workflow has been red on every push since February. The action never loaded the repo's markdown-link-check-config.json, and the README contains 38 links that return 404/410. This PR fixes both and hardens the workflow. The remaining noise (403s from datacenter blocking, timeouts, 429s) is not addressed here.

• Workflow config

  • Passes config-file: markdown-link-check-config.json. The action defaults to mlc_config.json, so the custom User-Agent and ignore patterns were never applied in CI.
  • Adds permissions: contents: read.
  • Pins actions/checkout (v4.2.2) and gaurav-nelson/github-action-markdown-link-check (v1) to commit SHAs, with version comments.
  • Removes the commented-out legacy workflow block.

  permissions:
    contents: read
  ...
  - uses: gaurav-nelson/github-action-markdown-link-check@5c5dfc0a… # v1
    with:
      config-file: markdown-link-check-config.json

• README links

  • Updates metager3.de/en/ to metager.de/ and technisette.com/p/home to technisette.com/.
  • Removes the entries for the other 36 dead URLs. Removal is by exact markdown link target, so a URL listed twice loses both entries. This affects Exif Viewer and the Global Terrorism Database.
  • facebook.com/directory/people/ is kept, since it is a different URL from the dead directory/.
  • The README's original line endings are preserved.

Reviewer note: removal was chosen over replacement because the issue reports these as permanently gone. If you would rather keep any of them, for example via an archive link, say so.

• Fixes Link check workflow fails on every push: config file never applied, 38 links verified dead #200