refactor: migrate agent system from XML tool calling to OpenAI SDK with native function calling

- Replace custom XML-based tool parsing (XMLToolExecutor) with OpenAI SDK's
  native tool_calls via `openai` npm package (dangerouslyAllowBrowser)
- Consolidate 4 LLM providers (OpenAI, Claude, Gemini, ClaudeOpenRouter)
  into a single OpenAI SDK-based LLMProvider compatible with any
  OpenAI-style API (OpenAI, OpenRouter, Ollama, vLLM)
- Move agentic tool execution loop from ChatBox into AgentCore
- Update Message type to support tool roles, tool_calls, and tool_call_id
- Update system prompt to remove XML formatting instructions (~45% smaller)
- Remove AttemptCompletionTool (replaced by stop_reason detection),
  ThinkTool, and ThinkingTool
- Polish tool descriptions for OpenAI function calling schema compliance
- Normalize base URLs by stripping /chat/completions suffix

src/components/settings/sections/GeneralSettings.tsx

@@ -368,7 +368,7 @@ const GeneralSettings: React.FC = () => {
             <input 
               type="text" 
               className="settings-input"
-              placeholder="e.g. https://openrouter.ai/api/v1/chat/completions"
+              placeholder="e.g. https://openrouter.ai/api/v1"
               value={claudeOpenRouterBaseUrl}
               onChange={(e) => handleClaudeOpenRouterBaseUrlChange(e.target.value)}
             />
@@ -424,7 +424,7 @@ const GeneralSettings: React.FC = () => {
             <input 
               type="text" 
               className="settings-input"
-              placeholder="e.g. https://openrouter.ai/api/v1/chat/completions"
+              placeholder="e.g. https://openrouter.ai/api/v1"
               value={compatibleBaseUrl}
               onChange={(e) => handleCompatibleBaseUrlChange(e.target.value)}
             />