docs: add observability phase to nix-cache plan

- Add Phase 6 for alerting and Grafana dashboards
- Document available Prometheus metrics
- Include example alerting rules for build failures

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

docs/plans/nix-cache-reprovision.md

@@ -118,6 +118,30 @@ Or trigger builds from CI after merges to master.
 - **Disk size?** 200GB for new host
 - **Build host specs?** 8 cores, 16-24GB RAM matches current nix-cache01
 
+### Phase 6: Observability
+
+1. **Alerting rules** for build failures:
+   ```promql
+   # Alert if any build fails
+   increase(homelab_deploy_build_host_total{status="failure"}[1h]) > 0
+
+   # Alert if no successful builds in 24h (scheduled builds stopped)
+   time() - homelab_deploy_build_last_success_timestamp > 86400
+   ```
+
+2. **Grafana dashboard** for build metrics:
+   - Build success/failure rate over time
+   - Average build duration per host (histogram)
+   - Build frequency (builds per hour/day)
+   - Last successful build timestamp per repo
+
+Available metrics:
+- `homelab_deploy_builds_total{repo, status}` - total builds by repo and status
+- `homelab_deploy_build_host_total{repo, host, status}` - per-host build counts
+- `homelab_deploy_build_duration_seconds_{bucket,sum,count}` - build duration histogram
+- `homelab_deploy_build_last_timestamp{repo}` - last build attempt
+- `homelab_deploy_build_last_success_timestamp{repo}` - last successful build
+
 ## Open Questions
 
 - [ ] When to cut over DNS from nix-cache01 to nix-cache02?