Implement auth_init() in auth.h to detect API keys from environment variables (R_KEY or OPENAI_API_KEY) and set auth_type accordingly. In main.c, call auth_init() during init() and replace the conditional commercial/free version startup messages with a simple carriage return clear.
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.
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.
