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

public/chat/error_no_openai_compatible_base_url.md

@@ -4,6 +4,6 @@ You selected OpenAI Compatible as your LLM Provider, but no Base URL is configur
 
 How to fix:
 - Go to **Settings ⚙️ → General → OpenAI Compatible Server**
-- Enter the **Base URL** for your provider (e.g., `https://openrouter.ai/api/v1/chat/completions` for OpenRouter, `http://localhost:11434/api/chat` for Ollama, etc.)
+- Enter the **Base URL** for your provider (e.g., `https://openrouter.ai/api/v1` for OpenRouter, `http://localhost:11434/api/chat` for Ollama, etc.)
 
 After updating your settings, try your request again.