Add AUTH_TYPE enum and global auth_type variable to auth.h, replacing the FREE_VERSION preprocessor conditional in chat.h with a dynamic get_prompt_model() function that returns "gpt-3.5-turbo" for free users and "gpt-4o-mini" for API key holders. Update all references to prompt_model to use the new function, enabling runtime model switching without recompilation.
Implement tool calling infrastructure including tools.h with http_fetch function description and execution, message_add_tool_call and message_add_tool_result helpers in messages.h, tool descriptions injection into user messages and chat_json, http header parsing utilities in http.h, curl_get function in http_curl.h, and tool call execution loop in openai_chat that recursively processes tool results.
Add initial project structure including .gitignore for build artifacts,
Makefile with gcc build and run targets, README placeholder, cgi-bin
directory with gpt.py and gpt_template.html, and core C headers for
chat, http, line input, markdown rendering, and messages.
