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/litellm/chunks_paths/paths_litellm.json

@@ -0,0 +1,20 @@
+{
+  "/litellm/.well-known/litellm-ui-config": {
+    "get": {
+      "summary": "Get Ui Config",
+      "operationId": "get_ui_config_litellm__well_known_litellm_ui_config_get",
+      "responses": {
+        "200": {
+          "description": "Successful Response",
+          "content": {
+            "application/json": {
+              "schema": {
+                "$ref": "#/components/schemas/UiDiscoveryEndpoints"
+              }
+            }
+          }
+        }
+      }
+    }
+  }
+}