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/invalid.integration.yaml

@@ -0,0 +1,19 @@
+schema_version: open-reuse.integration.v0.1
+id: INVALID_ID
+name: Fixture Invalid Integration
+
+upstream:
+  name: Example Upstream
+
+reuse:
+  primary_reuse_mode: not-a-real-mode
+
+boundary:
+  type: adapter
+
+validation:
+  harness: echo ok
+
+maintenance:
+  escalation_conditions:
+    - validation failure