feat: migrate Loki to community Monolithic chart

[systemdbrew]

Summary

• migrate Loki from the legacy Grafana chart repository to the Grafana Community chart
• pin Loki Helm chart 18.13.1
• rename deployment mode from deprecated SingleBinary to Monolithic
• use TSDB v13 with filesystem storage for the single-replica homelab/reference deployment
• persist Loki data on longhorn-standard with explicit PVC retention on delete/scale-down
• keep non-monolithic component replicas disabled
• allow the Grafana Community Helm repository in the Argo CD AppProject
• update the dependency/deployability audit and correct its audit date

Validation / upstream review

The migration follows Grafana's current Monolithic Helm guidance: one replica requires commonConfig.replication_factor: 1, the community chart uses deploymentMode: Monolithic, and the chart still deploys the loki-gateway service used by Alloy. The release pin is the published loki-18.13.1 chart.

Filesystem storage is intentional for this sanitized small-cluster reference and is documented as non-HA. External object storage remains the recommendation for larger/HA Loki deployments.

Follow-up before merge

Render the exact chart with the committed values and verify the gateway service/PVC fields. The temporary README noreply-test marker should also be removed before merge.