build: add -Wshadow -Wconversion -Wstrict-prototypes -O2

Tighten compiler warnings to catch implicit conversions, shadowed
variables, and missing prototypes. Enable -O2 optimization for
release builds.

Amp-Thread-ID: https://ampcode.com/threads/T-019cfc7e-c61f-7459-9d48-5c06613d5c89
Co-authored-by: Amp <amp@ampcode.com>

Author: vinitkumar

c/Makefile

@@ -12,7 +12,7 @@ else
 endif
 
 CLANG := clang
-CFLAGS_COMMON = -Wall -Wextra -pedantic
+CFLAGS_COMMON = -Wall -Wextra -Wpedantic -Wshadow -Wconversion -Wstrict-prototypes -O2
 SRC = src/chatbot.c
 
 # Default targets