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_completions.json

@@ -0,0 +1,55 @@
+{
+  "/completions": {
+    "post": {
+      "tags": [
+        "completions"
+      ],
+      "summary": "Completion",
+      "description": "Follows the exact same API spec as `OpenAI's Completions API https://platform.openai.com/docs/api-reference/completions`\n\n```bash\ncurl -X POST http://localhost:4000/v1/completions \n-H \"Content-Type: application/json\" \n-H \"Authorization: Bearer sk-1234\" \n-d '{\n    \"model\": \"gpt-3.5-turbo-instruct\",\n    \"prompt\": \"Once upon a time\",\n    \"max_tokens\": 50,\n    \"temperature\": 0.7\n}'\n```",
+      "operationId": "completion_completions_post",
+      "security": [
+        {
+          "APIKeyHeader": []
+        }
+      ],
+      "parameters": [
+        {
+          "name": "model",
+          "in": "query",
+          "required": false,
+          "schema": {
+            "anyOf": [
+              {
+                "type": "string"
+              },
+              {
+                "type": "null"
+              }
+            ],
+            "title": "Model"
+          }
+        }
+      ],
+      "responses": {
+        "200": {
+          "description": "Successful Response",
+          "content": {
+            "application/json": {
+              "schema": {}
+            }
+          }
+        },
+        "422": {
+          "description": "Validation Error",
+          "content": {
+            "application/json": {
+              "schema": {
+                "$ref": "#/components/schemas/HTTPValidationError"
+              }
+            }
+          }
+        }
+      }
+    }
+  }
+}