docs: add host cleanup plan for decommissioned hosts

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

docs/plans/host-cleanup.md

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+# Host Cleanup
+
+## Overview
+
+Remove decommissioned/unused host configurations that are no longer reachable on the network.
+
+## Hosts to review
+
+The following hosts return "no route to host" from Prometheus scraping and are likely no longer needed:
+
+- `media1` (10.69.12.82)
+- `ns3` (10.69.13.7)
+- `ns4` (10.69.13.8)
+- `nixos-test1` (10.69.13.10)
+
+## Steps
+
+1. Confirm each host is truly decommissioned (not just temporarily powered off)
+2. Remove host directory from `hosts/`
+3. Remove `nixosConfigurations` entry from `flake.nix`
+4. Remove host's age key from `.sops.yaml`
+5. Remove per-host secrets from `secrets/<hostname>/` if any
+6. Verify DNS zone and Prometheus targets no longer include the removed hosts after rebuild