docs: update nix-cache-reprovision plan with progress

- Mark Phase 1 (new build host) and Phase 2 (NATS build triggering) complete
- Document nix-cache02 configuration and tested build times
- Add remaining work for Harmonia, Actions runner, and DNS cutover
- Enable --enable-builds flag in MCP config

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

.mcp.json

@@ -31,7 +31,8 @@
         "--",
         "mcp",
         "--nats-url", "nats://nats1.home.2rjus.net:4222",
-        "--nkey-file", "/home/torjus/.config/homelab-deploy/test-deployer.nkey"
+        "--nkey-file", "/home/torjus/.config/homelab-deploy/test-deployer.nkey",
+        "--enable-builds"
       ]
     },
     "git-explorer": {