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_messages.json

@@ -0,0 +1,52 @@
+{
+  "/v1/messages": {
+    "post": {
+      "tags": [
+        "[beta] Anthropic `/v1/messages`"
+      ],
+      "summary": "Anthropic Response",
+      "description": "Use `{PROXY_BASE_URL}/anthropic/v1/messages` instead - [Docs](https://docs.litellm.ai/docs/pass_through/anthropic_completion).\n\nThis was a BETA endpoint that calls 100+ LLMs in the anthropic format.",
+      "operationId": "anthropic_response_v1_messages_post",
+      "responses": {
+        "200": {
+          "description": "Successful Response",
+          "content": {
+            "application/json": {
+              "schema": {}
+            }
+          }
+        }
+      },
+      "security": [
+        {
+          "APIKeyHeader": []
+        }
+      ]
+    }
+  },
+  "/v1/messages/count_tokens": {
+    "post": {
+      "tags": [
+        "[beta] Anthropic Messages Token Counting"
+      ],
+      "summary": "Count Tokens",
+      "description": "Count tokens for Anthropic Messages API format.\n\nThis endpoint follows the Anthropic Messages API token counting specification.\nIt accepts the same parameters as the /v1/messages endpoint but returns\ntoken counts instead of generating a response.\n\nExample usage:\n```\ncurl -X POST \"http://localhost:4000/v1/messages/count_tokens?beta=true\"       -H \"Content-Type: application/json\"       -H \"Authorization: Bearer your-key\"       -d '{\n    \"model\": \"claude-3-sonnet-20240229\",\n    \"messages\": [{\"role\": \"user\", \"content\": \"Hello Claude!\"}]\n  }'\n```\n\nReturns: {\"input_tokens\": <number>}",
+      "operationId": "count_tokens_v1_messages_count_tokens_post",
+      "responses": {
+        "200": {
+          "description": "Successful Response",
+          "content": {
+            "application/json": {
+              "schema": {}
+            }
+          }
+        }
+      },
+      "security": [
+        {
+          "APIKeyHeader": []
+        }
+      ]
+    }
+  }
+}