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>

src/ci_orchestrator/commands/vm.py

@@ -194,9 +194,9 @@ def _list_orphans(clone_base: Path, max_age_hours: int) -> list[Path]:
     cutoff = datetime.now(tz=UTC) - timedelta(hours=max_age_hours)
     out: list[Path] = []
     for entry in clone_base.iterdir():
-        if not entry.is_dir():
-            continue
         try:
+            if not entry.is_dir():
+                continue
             mtime = datetime.fromtimestamp(entry.stat().st_mtime, tz=UTC)
         except OSError:
             continue