Fix readline path resolution for Apple Silicon

[petrostrak]

Fix readline path resolution for Apple Silicon (CI + build docs)

• Both CI pipelines and the "Building from source" docs hardcoded the Intel-Mac Homebrew prefix (/usr/local/opt/readline) for readline's include/lib paths. On Apple Silicon, Homebrew installs to /opt/homebrew, so that path silently doesn't exist and the build falls back to Apple's SDK readline.h (a libedit compatibility shim), causing rl_resize_terminal and related symbols to be undeclared.
• .github/workflows/build-osx.yaml: removed the hardcoded CPPFLAGS/LDFLAGS and resolve the readline prefix dynamically via brew --prefix readline, so it works on both arm64 and x86_64 runners.
• .github/workflows/c-cpp.yml: added libreadline-dev to the apt-get install step — the Linux job was missing the readline dev headers entirely, so configure fell back to a minimal extern declaration missing rl_point, rl_line_buffer, rl_display_prompt, rl_catch_signals, etc.
• README.md: updated the Mac build instructions to use $(brew --prefix readline) instead of the hardcoded path, so the documented steps work on any Mac architecture.

Test plan

• Local macOS (Apple Silicon) build verified with corrected CPPFLAGS/LDFLAGS
• Local linux build verified
• CI green on push (both C build (OSX) and C/C++ CI workflows)