C2(deploy-infra-alerting): impl fix TextfileStale to always return data

Query all textfile mtimes (time() - node_textfile_mtime_seconds) and
threshold at > 3600s, instead of filtering with > 3600 which returns
empty results when everything is fresh.

This means:
- Fresh textfiles: query returns low values, threshold not met → OK
- Stale textfiles: query returns high values, threshold met → Alerting
- Missing textfiles: series vanishes, noDataState=Alerting → Alerting

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

argocd/manifests/grafana/alerting.yaml

@@ -93,7 +93,7 @@ groups:
         title: TextfileStale
         condition: C
         for: 15m
-        noDataState: OK
+        noDataState: Alerting
         execErrState: Alerting
         annotations:
           summary: >-
@@ -110,7 +110,7 @@ groups:
               to: 0
             model:
               expr: >-
-                time() - node_textfile_mtime_seconds > 3600
+                time() - node_textfile_mtime_seconds
               interval: ""
               refId: A
           - refId: B
@@ -137,7 +137,7 @@ groups:
                 - evaluator:
                     type: gt
                     params:
-                      - 0
+                      - 3600
                   operator:
                     type: and
               refId: C