feat: add cwd attribute to MCP server config
Adds optional "cwd" field to mcp.json server configurations to set the
working directory for stdio MCP servers.
- Add cwd field to mcp_server_config struct
- Unix: call chdir() before execvp() in child process
- Windows: pass lpCurrentDirectory to CreateProcessA()
- Update mcp_config.example.json with usage example
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
