modules: add homelab.host for host metadata

Add a shared `homelab.host` module that provides host metadata for
multiple consumers:
- tier: deployment tier (test/prod) for future homelab-deploy service
- priority: alerting priority (high/low) for Prometheus label filtering
- role: primary role of the host (dns, database, monitoring, etc.)
- labels: free-form labels for additional metadata

Host configurations updated with appropriate values:
- ns1, ns2: role=dns with dns_role labels
- nix-cache01: priority=low, role=build-host
- vault01: role=vault
- jump: role=bastion
- template, template2, testvm01, vaulttest01: tier=test, priority=low

The module is now imported via commonModules in flake.nix, making it
available to all hosts including minimal configurations like template2.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/plans/nats-deploy-service.md

@@ -240,6 +240,9 @@ All NKeys stored in Vault - MCP gets limited credentials, admin CLI gets full-ac
 
 Rather than defining `tier` in the listener config, use a central `homelab.host` module that provides host metadata for multiple consumers. This aligns with the approach proposed in `docs/plans/prometheus-scrape-target-labels.md`.
 
+**Status:** The `homelab.host` module is implemented in `modules/homelab/host.nix`.
+Hosts can be filtered by tier using `config.homelab.host.tier`.
+
 **Module definition (in `modules/homelab/host.nix`):**
 ```nix
 homelab.host = {

docs/plans/prometheus-scrape-target-labels.md

@@ -58,6 +58,9 @@ This implementation uses a shared `homelab.host` module that provides host metad
 
 ### 1. Create `homelab.host` module
 
+**Status:** Step 1 (Create `homelab.host` module) is complete. The module is in
+`modules/homelab/host.nix` with tier, priority, role, and labels options.
+
 Create `modules/homelab/host.nix` with shared host metadata options:
 
 ```nix