Add CodeQL, Codecov, Codacy, Renovate and actionlint

- CodeQL analyses the workflows and the C++ sources, without a build,
  with the security-extended queries, on every push to main, every pull
  request and once a week.
- A coverage job runs the tests under Clang's own coverage on Linux and
  a job of its own uploads the lcov report to Codecov through OIDC.
- .codacy.yml leaves out the output of Bazel and the rules_cc patch.
- Renovate proposes Bazel modules, Bazel itself and GitHub Actions once
  a week, a week after their release; rules_cc waits for approval on
  the dashboard, since its clang-cl patch has to be checked first.
- actionlint joins zizmor in Workflow Security.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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