docs(proxmox): Document all six PBS GC verdict shapes

The Liveness Check section now documents ALL SIX verdict shapes exactly
as emitted by proxmox-monitor.sh:

1. ✅ PBS GC: healthy (last run Nh ago, pending-bytes: N B)
2. 🔴 PBS GC: stale (last run Nh ago, pending-bytes: N B)
3. 🔴 PBS GC: probe-failed: storepve:192.168.68.6 (expected JSON, got 000)
4. 🔴 PBS GC: probe-failed: storepve:192.168.68.6 (unparseable JSON)
5. 🔴 PBS GC: never-run (storepve-datastore not found in GC list)
6. 🔴 PBS GC: never-run (storepve-datastore has no last-run-endtime)

Fixed the quoted healthy example (line ~114) to include the pending-bytes
suffix the code now appends. Previously the prose only documented the
probe-failed shape, missing the PR's own headline cases (never-run).

Proof: grep -n 'never-run' proxmox-monitor.prose.md now returns two lines
(lines 106 and 108), documenting both never-run variants.

Branch: fix/pbs-gc-liveness-signal-20260919

proxmox-monitor.prose.md

@@ -110,8 +110,26 @@ The new `proxmox-monitor.sh` leg checks storepve-datastore GC health:
 - Reads GC state from CT 107: `pct exec 107 -- proxmox-backup-manager garbage-collection list --output-format json`
 - **FAILS** if `last-run-endtime` is older than 48 hours
 - Reports age in hours and pending-bytes
-- A failed probe (000/timeout) is reported as `probe-failed: storepve:192.168.68.6 (expected JSON, got 000)` — **never** rendered as "GC stale"
-- The healthy branch prints an explicit healthy line: `✅ PBS GC: healthy (last run Xh ago)`
+
+**All six verdict shapes** (exactly as emitted by the script):
+
+1. **Healthy** (fresh GC, 0 B pending):
+   `✅ PBS GC: healthy (last run 1h ago, pending-bytes: 0 B)`
+
+2. **Stale** (GC ran >48h ago):
+   `🔴 PBS GC: stale (last run 49h ago, pending-bytes: 1048576 B)`
+
+3. **Probe-failed: empty read** (000/timeout/unreadable):
+   `🔴 PBS GC: probe-failed: storepve:192.168.68.6 (expected JSON, got 000)`
+
+4. **Probe-failed: unparseable** (non-empty but invalid JSON — the "command not found" case):
+   `🔴 PBS GC: probe-failed: storepve:192.168.68.6 (unparseable JSON)`
+
+5. **Never-run: datastore absent** (valid JSON but storepve-datastore not in list):
+   `🔴 PBS GC: never-run (storepve-datastore not found in GC list)`
+
+6. **Never-run: no endtime** (valid JSON with datastore present but last-run-endtime is null/0):
+   `🔴 PBS GC: never-run (storepve-datastore has no last-run-endtime)`
 
 ## Operations