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/.env.example

@@ -0,0 +1,14 @@
+# =============================================================================
+# sample room — Copy to .env and customize
+# =============================================================================
+
+DEPLOYMENT_NAME=sample
+NETWORK_NAME=sample_network
+
+BACKEND_PORT=8120
+FRONTEND_PORT=3120
+DB_PORT=5532
+
+POSTGRES_DB=fixture
+POSTGRES_USER=postgres
+POSTGRES_PASSWORD=localdev123