ci: split orchestrator integrity into parallel jobs for faster validation

Rewrite the monolith orchestrator-integrity.yml (1110 lines, single job,
3+ hour sequential execution) into 4 parallel jobs that run on separate
runners:

- k8s-tests: k3d cluster + LocalStack, 5 tests
- aws-provider-tests: LocalStack only, 10 tests
- local-docker-tests: Docker + LocalStack for S3 tests, 9 tests
- rclone-tests: rclone + LocalStack, 1 test

Key improvements:
- Wall-clock time drops from ~3h to ~1h (longest single job)
- Disk exhaustion eliminated: each job gets its own fresh 14GB runner
- Cleanup logic deduplicated via sourced shell functions instead of
  15 copy-pasted 30-line blocks
- K3d node image cleanup only runs in the k8s job (where it matters)
- Light cleanup (cache + docker prune -f) between tests; heavy cleanup
  (prune -af --volumes) only at job boundaries
- workflow_call interface unchanged; integrity-check.yml needs no changes

Ref: #794

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: frostebite