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-status.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+# Show what's running on the shared `spr` cluster.
+set -e
+
+if ! kind get clusters 2>/dev/null | grep -q '^spr$'; then
+    echo "No 'spr' kind cluster — run ctrl/kind-up.sh"
+    exit 0
+fi
+
+kubectl --context kind-spr get namespaces -l soleprint-room
+echo
+kubectl --context kind-spr get pods -A -l soleprint-room