feat: B5 — port retention/template-backup to Python, deploy systemd timers

- Add `retention run` command (ports Invoke-RetentionPolicy.ps1): purges
  old artifact/log dirs with aggressive mode when disk space is low.
- Add `template backup` command (ports Backup-CITemplate.ps1): 7z -mx=1
  compressed archives in /var/lib/ci/backups/, prunes to keep-count=3,
  stops/restarts act-runner.service around the copy.
- Update ci-retention-policy.service and ci-backup-template.service to use
  Python; pwsh is no longer required on the Linux host.
- Fix ci-watch-runner-health.service: pass --service-name act-runner
  (Linux service name, not Windows act_runner default).
- Fix _list_orphans in vm.py: wrap is_dir() inside the OSError try block
  so a stat() failure on an entry is silently skipped rather than raised.
- Mark B5 complete in PhaseB-user-checklist.md.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

deploy/systemd/ci-watch-runner-health.service

@@ -10,7 +10,7 @@ Group=ci-runner
 EnvironmentFile=/etc/ci/environment
 # `monitor runner` enforces internal rate-limit: max 3 restarts/h
 # (matches Windows CI-RunnerHealth scheduled task semantics).
-ExecStart=/opt/ci/venv/bin/python -m ci_orchestrator monitor runner
+ExecStart=/opt/ci/venv/bin/python -m ci_orchestrator monitor runner --service-name act-runner
 TimeoutStartSec=5min
 Nice=15
 ProtectSystem=strict