fix: untrack host-disk-bands.json and document gitignored status

The state file is runtime state (rewritten every scan), so tracking it in git
means:
- every executor's clone becomes permanently dirty after one run
- a scan in one clone produces a merge conflict with a scan in another
- the committed baseline can be stale in a way nobody notices

Added to .gitignore and removed from the index. Contract updated to say
'the state file lives at <abs path> and is gitignored runtime state - the
scanner creates it on first run'.

.gitignore

@@ -1 +1,2 @@
 __pycache__/
+state/host-disk-bands.json

disk-gc-threat-response.prose.md

@@ -65,7 +65,7 @@ Host filesystems have their own risk profile and their own bands. A host root ne
 
 **Escalations are STATE-CHANGE driven, not per-run.** A volume alerts ONCE when it enters a higher band (GREEN->WARN, WARN->AMBER, AMBER->RED) and ONCE when it drops back down (a recovery notice). While a volume stays in the same band, it is reported in the scan output only — no DM, no channel alert. This prevents the same 96% easystore2 from re-DMing the owner on every 6-hour scan and drowning a real warning in noise.
 
-**State lives in a small JSON state file:** the scanner resolves it to an absolute path from the script's location — `$(dirname "$0")/state/host-disk-bands.json` (i.e., the `state/` directory next to the `scripts/` directory in the prose-contracts repo). Keyed by `host/volume` → last-seen band. The scanner reads the prior band, compares to the current band, and DMs only on a transition. The state file is written after every scan. (Chosen over a periodic digest because the scan already runs every 6h and a transition is genuinely new, actionable state that warrants an immediate DM — but only once.)
+**State lives in a small JSON state file:** the scanner resolves it to an absolute path from the script's location — `$(dirname "$0")/state/host-disk-bands.json` (i.e., the `state/` directory next to the `scripts/` directory in the prose-contracts repo). The state file is gitignored runtime state — the scanner creates it on first run. Keyed by `host/volume` → last-seen band. The scanner reads the prior band, compares to the current band, and DMs only on a transition. The state file is written after every scan. (Chosen over a periodic digest because the scan already runs every 6h and a transition is genuinely new, actionable state that warrants an immediate DM — but only once.)
 
 **Volume naming rule:** Every host line MUST name the volume and what lives on it. Example output:
 

state/host-disk-bands.json

@@ -1,18 +0,0 @@
-{
-  "acerpve//": "GREEN",
-  "acerpve//media/mediastore": "GREEN",
-  "acerpve//media/storage": "GREEN",
-  "amdpve//": "GREEN",
-  "amdpve//media/mediastore": "GREEN",
-  "amdpve//media/storage": "GREEN",
-  "storepve//": "GREEN",
-  "storepve//media/easystore2": "HOST-RED",
-  "storepve//media/mediastore": "GREEN",
-  "storepve//media/reanim": "HOST-WARN",
-  "storepve//media/storage": "GREEN",
-  "minipve//": "GREEN",
-  "minipve//media/storage": "GREEN",
-  "ocupve//": "GREEN",
-  "ocupve//media/mediastore": "GREEN",
-  "ocupve//media/storage": "GREEN"
-}