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/welcome_new.md

@@ -7,7 +7,7 @@ Welcome to **K.G.Studio Musician Assistant** — your in‑browser AI partner fo
 - In **Settings ⚙️ → General → LLM Provider**, select **OpenAI Compatible**.
 - In **OpenAI Compatible Server → Key**, paste your key. Note: on non‑localhost, your key isn’t persisted; you’ll re‑enter it after refresh. Keep it safe.
 - In **OpenAI Compatible Server → Model**, enter `qwen/qwen3-30b-a3b:free`. Note: non‑free models may require billing.
-- In **OpenAI Compatible Server → Base URL**, enter `https://openrouter.ai/api/v1/chat/completions`.
+- In **OpenAI Compatible Server → Base URL**, enter `https://openrouter.ai/api/v1`.
 
 *(Alternatively, you can use the official OpenAI API, or any OpenAI‑compatible service)*