feat: implement PBS GC four-state logic and update contracts

- Implement four-state PBS GC logic in proxmox-monitor.sh:
  - probe-failed: unparseable JSON, store not found, or empty body → FAIL
  - running: collection in progress (last-run-endtime absent, upid present) → DO NOT FAIL
  - stale: no completed run within 48h → FAIL, naming last completed run age
  - healthy: completed within 48h → PASS, naming endtime and pending bytes

- Add tests/test_pbs_gc_states.sh covering all four states
  - Proves the test bites on the pre-fix version (5/6 tests fail)
  - All 6 tests pass against the fixed version

- Update contract-run.sh to map disk-gc-threat-response -> scripts/disk-gc-scan.py

- Update Execution sections of host-scheduled contracts:
  infrastructure-monitoring, zulip-health, litellm-health, agent-health-check,
  disk-gc-threat-response, pm2-self-heal
  Adding note that execution is host-scheduled via cron, not agent session ack.

scripts/contract-run.sh

@@ -12,7 +12,7 @@
 #   zulip-health              -> scripts/zulip-monitor.sh
 #   agent-health-check        -> scripts/agent-health-check.py
 #   litellm-health            -> scripts/litellm-health-check.py
-#   disk-gc-threat-response   -> (Python script, needs investigation)
+#   disk-gc-threat-response   -> scripts/disk-gc-scan.py
 #   pm2-self-heal             -> scripts/pm2-self-heal.sh
 #
 # Exit codes: