feat(ci): unify CI and local integration test infrastructure

- Replace tests/infrastructure/Dockerfile with tests/Dockerfile.test
  (single multi-stage Dockerfile for both CI and local dev)
- CI container-image job now builds from Dockerfile.test target dev-infra
- Add ARM support: PowerShell installed via dotnet tool on arm64,
  APT package on amd64
- Replace tests/dev.sh (bash) with tests/dev.ps1 (PowerShell) for
  cross-platform support (Windows, macOS, Linux)
- Add -DockerHost parameter for running x86 containers on a remote
  Docker host from ARM Macs (rsyncs repo, uses SSH Docker transport)
- Add -NoCleanup switch to keep nested PVE VMs after integration tests
- integration command now provisions nested PVE VMs instead of testing
  against a pre-existing PVE directly
- Share /opt/pve-isos host path between CI and local dev (was separate
  Docker named volume)
- Delete tools/Invoke-Tests.ps1 (unused, overlapped with run-integration.sh)
- Add .gitignore entries for Terraform state/artifacts
- Update all documentation references

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -15,3 +15,7 @@ BenchmarkDotNet.Artifacts/
 .env.*
 !.env.*.example
 tests/.env.test
+tests/infrastructure/.terraform/
+tests/infrastructure/.terraform.lock.hcl
+tests/infrastructure/terraform.tfstate*
+tests/infrastructure/*.tfvars.json