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>

ctrl/kind-down.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+# Delete the shared `spr` kind cluster (drops every room's namespace too).
+# Use `gen/<room>/ctrl/k8s-down.sh` instead if you only want to remove
+# a single room's namespace.
+set -e
+
+if kind get clusters 2>/dev/null | grep -q '^spr$'; then
+    echo "Deleting kind cluster 'spr'..."
+    kind delete cluster --name spr
+else
+    echo "No kind cluster 'spr' to delete."
+fi