feat(ecosystem): adopt hub-core dependency and CI pin gate (CORE-WP-0009)

Import slugify_or_default from hub-core, add contract tests, vendor hub-core in
Docker/Forgejo CI, document metadata isolation and pagination deferral, and
align INTENT/SCOPE with three-repo stack.

scripts/check_staging_profile.py

@@ -80,7 +80,7 @@ def main() -> int:
 
     if dockerfile.exists():
         docker_text = dockerfile.read_text(encoding="utf-8")
-        for needle in ["uv sync --no-dev --frozen", "EXPOSE 8010", "CORE_HUB_AUTO_CREATE_TABLES"]:
+        for needle in ["hub_core_src", "EXPOSE 8010", "CORE_HUB_AUTO_CREATE_TABLES"]:
             if needle not in docker_text:
                 failures.append(f"Dockerfile missing: {needle}")