feat(cli): add open-reuse validate and register portfolio integrations

Implement Integration Definition validator CLI with schema and index checks,
pytest suite, and CI workflow. Register open-cmis-tck and issue-core-gitea in
the integration index.

Closes OPEN-WP-0003 and OPEN-WP-0004.

tests/fixtures/valid.integration.yaml

@@ -0,0 +1,26 @@
+schema_version: open-reuse.integration.v0.1
+id: fixture-valid
+name: Fixture Valid Integration
+status: registered
+owner: open-reuse
+
+upstream:
+  name: Example Upstream
+  project_url: https://example.com/upstream
+
+reuse:
+  primary_reuse_mode: adapter
+
+boundary:
+  type: adapter
+  local_adapter: fixture.adapter.Adapter
+  reused_surface: upstream API
+
+validation:
+  harness: python3 -m pytest tests/
+
+maintenance:
+  maintainers:
+    - fixture-team
+  escalation_conditions:
+    - validation failure