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__v1_model.json

@@ -0,0 +1,55 @@
+{
+  "/v1/model/info": {
+    "get": {
+      "tags": [
+        "model management"
+      ],
+      "summary": "Model Info V1",
+      "description": "Provides more info about each model in /models, including config.yaml descriptions (except api key and api base)\n\nParameters:\n    litellm_model_id: Optional[str] = None (this is the value of `x-litellm-model-id` returned in response headers)\n\n    - When litellm_model_id is passed, it will return the info for that specific model\n    - When litellm_model_id is not passed, it will return the info for all models\n\nReturns:\n    Returns a dictionary containing information about each model.\n\nExample Response:\n```json\n{\n    \"data\": [\n                {\n                    \"model_name\": \"fake-openai-endpoint\",\n                    \"litellm_params\": {\n                        \"api_base\": \"https://exampleopenaiendpoint-production.up.railway.app/\",\n                        \"model\": \"openai/fake\"\n                    },\n                    \"model_info\": {\n                        \"id\": \"112f74fab24a7a5245d2ced3536dd8f5f9192c57ee6e332af0f0512e08bed5af\",\n                        \"db_model\": false\n                    }\n                }\n            ]\n}\n\n```",
+      "operationId": "model_info_v1_v1_model_info_get",
+      "security": [
+        {
+          "APIKeyHeader": []
+        }
+      ],
+      "parameters": [
+        {
+          "name": "litellm_model_id",
+          "in": "query",
+          "required": false,
+          "schema": {
+            "anyOf": [
+              {
+                "type": "string"
+              },
+              {
+                "type": "null"
+              }
+            ],
+            "title": "Litellm Model Id"
+          }
+        }
+      ],
+      "responses": {
+        "200": {
+          "description": "Successful Response",
+          "content": {
+            "application/json": {
+              "schema": {}
+            }
+          }
+        },
+        "422": {
+          "description": "Validation Error",
+          "content": {
+            "application/json": {
+              "schema": {
+                "$ref": "#/components/schemas/HTTPValidationError"
+              }
+            }
+          }
+        }
+      }
+    }
+  }
+}