feat(llm): mask API keys in UI responses and prevent masked-key leakage in fetch/test/save payloads

- Add mask_api_key() and is_masked_or_placeholder() to llm_provider service
- Return masked keys in all provider CRUD endpoints
- Reject masked/placeholder keys in fetch_models and test_provider_config
- Show masked key with Change button in ProviderConfig.svelte edit form
- Exclude masked keys from fetch-models, test, and submit payloads on frontend
- Update semantics-core skill with clarified complexity tier rules
- Switch agent modes from subagent to all

docs/openapi/superset/components/securitySchemes.json

@@ -0,0 +1,12 @@
+{
+  "jwt": {
+    "bearerFormat": "JWT",
+    "scheme": "bearer",
+    "type": "http"
+  },
+  "jwt_refresh": {
+    "bearerFormat": "JWT",
+    "scheme": "bearer",
+    "type": "http"
+  }
+}\n