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/help.md

@@ -35,7 +35,7 @@ OpenRouter is a platform that provides unified access to a wide range of languag
     - `Anthropic: claude-4-sonnet` (`anthropic/claude-sonnet-4`: [Link](https://openrouter.ai/anthropic/claude-sonnet-4))
     - `Qwen: qwen3-30b-a3b` (FREE MODEL: `qwen/qwen3-30b-a3b:free`: [Link](https://openrouter.ai/qwen/qwen3-30b-a3b:free))
     - `Qwen: qwen3-235b-a22b` (FREE MODEL: `qwen/qwen3-235b-a22b:free`: [Link](https://openrouter.ai/qwen/qwen3-235b-a22b:free))
-6. Input the base URL `https://openrouter.ai/api/v1/chat/completions` of the OpenAI Compatible Server in **OpenAI Compatible Server → Base URL**.
+6. Input the base URL `https://openrouter.ai/api/v1` of the OpenAI Compatible Server in **OpenAI Compatible Server → Base URL**.
 
 ### Basic DAW Operations