feat(cfold): canonicalize custom test layout

tests/plausible/custom/test_health_check.py

@@ -0,0 +1,22 @@
+"""plausible — Phase-2 health_check."""
+
+from __future__ import annotations
+
+import os
+import sys
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "..", "runner"))
+from harness import http as harness_http  # noqa: E402
+
+
+def test_plausible_root_serves(live_app):
+    """GET /api/health → 200 (clickhouse+postgres ready).
+
+    `/` is NOT a reliable health probe (500s during datastore init; 302s to
+    /register once ready — and 500'd permanently under the pre-2026-06-11
+    62-char SECRET_KEY_BASE, see recipe_meta.EXTRA_ENV); the dedicated
+    /api/health endpoint is.
+    """
+    url = f"https://{live_app}/api/health"
+    status, _ = harness_http.retry_http_get(url, expect_status=(200,), max_wait=60, interval=3)
+    assert status == 200, f"GET {url} HTTP {status}"