add fixture-invoicing example, sample-room wrap, kind cluster support

- examples/fixture-invoicing/: FastAPI + Vue + Postgres demo (4-entity invoice fixture)
- cfg/sample/: wraps the fixture (managed.repos points at examples/)
- ctrl/kind-{up,down,status}.sh + per-room k8s render in soleprint/ctrl/k8s/
- build.py: relative repo paths, resilient rmtree, optional k8s render hook
- cfg/.gitignore: stop ignoring sample/ and standalone/ template rooms

Manifests render cleanly but kind cluster has not been run end-to-end yet.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cfg/sample/config.json

@@ -17,7 +17,14 @@
   },
   "veins": ["google"],
   "managed": {
-    "name": "sample"
+    "name": "sample",
+    "repos": {
+      "backend": "examples/fixture-invoicing/backend",
+      "frontend": "examples/fixture-invoicing/frontend"
+    }
+  },
+  "k8s": {
+    "enabled": true
   },
   "systems": [
     {