feat: substantial refactoring

[jackwrfuller]

Description

• Remove legacy govcms8 modules:
  • Delete govcms8_foundations (including govcms8_calendar_item, govcms8_layouts, govcms8_modifiers) and the duplicate top-level govcms8_layouts — these are Drupal 8 modules no longer used in D11
• Remove custom composer path repo:
  • Delete custom/composer/ (composer.json, patches.json) and the govcms/govcms-custom dependency, remove the path repository reference
• Clean up: Remove modules/README.md, .DS_Store, drupal/bartik dependency
• Simplify Dockerfile:
  • Remove COPY custom step, remove ScriptHandler.php copy (no longer needed at build time)
• Add CI pipeline:
  • GitHub Actions workflow that builds images with ahoy build, then integration tests against a fresh govCMS/scaffold (init, build, site install, homepage check)
• Harden ahoy build: Add set -e to ahoy build command so failures propagate correctly in CI.
• Remove the unusedrector.php.

[jackwrfuller]

keeping this one as draft for now, since it will need lots of testing of course.