feat: add Ollama support with conditional compilation and new build targets

Introduce conditional compilation via OLLAMA preprocessor flag to switch between OpenAI and Ollama API endpoints, model names, and URLs. Add `build_free` and `run_free` Makefile targets for the Ollama variant. Enhance REPL with `!models` command to list available models, improve error handling in `openai_process_chat_message`, and fix history line addition logic.

Makefile

@@ -1,4 +1,4 @@
-all: build build_rpylib run
+all: build build_free build_rpylib run_free
 
 # Variables for compiler and flags
 CC = gcc
@@ -9,9 +9,16 @@ build:
 	$(CC) main.c $(CFLAGS) -o r
 	publish r
 
+build_free:
+	$(CC) -DOLLAMA main.c $(CFLAGS) -o rf
+	publish rf
+
 build_rpylib:
 	$(CC) -shared -o rpylib.so -fPIC rpylib.c -lpython3.12 `python3-config --includes` -I/usr/include/CL -ljson-c -lcurl -lsqlite3
 	publish rpylib.so
 
 run: 
 	./r --verbose
+
+run_free:
+	./rf --verbose