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/.gitignore

@@ -10,6 +10,5 @@ __pycache__/
 *.pyc
 *.pyo
 
-# These are kept in main soleprint repo as templates
-standalone/
-sample/
+# standalone/ and sample/ are kept in the main soleprint repo as templates.
+# Other rooms (amar/, dlt/, etc.) are listed in the repo-root .gitignore.