feat: agent variants — migration, API, service integration, frontend, tests

- Migration 027: agent_variants table with single-active enforcement,
  variant_id column on agent_performance_log
- API: full CRUD, clone from agent/variant, activate/deactivate,
  per-variant performance metrics and history endpoints
- Services: extractor, event classifier, thesis rewriter all wired
  to AgentConfigResolver with variant override support
- Frontend: variant list, comparison view, create/edit/clone forms,
  activate/delete actions on Agents page
- Tests: API tests + 5 property-based tests (single-active invariant,
  clone preservation, config resolution, slug determinism, update idempotence)
- Spec files for agent-variants feature

services/extractor/client.py

@@ -263,6 +263,10 @@ class OllamaClient:
             },
         }
 
+        # Support context_window override via num_ctx (Requirement 10.4)
+        if self._config.context_window > 0:
+            payload["options"]["num_ctx"] = self._config.context_window
+
         url = f"{self._config.base_url}/api/chat"
         logger.info(
             "Ollama POST %s model=%s input_chars=%d",