feat: add CI examples for gating builds on axe Watcher results

[scottmries]

Summary

Adds results/ — copy-pasteable CI integrations that gate a build on the
axe Watcher accessibility threshold using the
axe-watcher-results CLI.
The framework examples in this repo (js/, java/) show how to run a
scan; these are one optional way to work with a run's results afterward:
retrieving them in CI and turning them into a pass/fail build gate.

results/
  README.md                      # gating model, exit-code table, install, secrets
  github-actions/a11y-gate.yml
  gitlab-ci/.gitlab-ci.yml
  shell/gate.sh                  # gate by commit SHA
  shell/session-results.sh       # retrieve/gate by session ID

Originally proposed in dequelabs/axe-watcher-results#46.

Verification

• prettier --check and shellcheck clean.
• Naming follows this repo's "axe Watcher" house style.

No QA required