docs: clarify public source build path

[Bortlesboat]

Summary

• add a short note that public source builds should use Xcode or the documented xcodebuild command
• clarify that the top-level ./build.sh script is for the private Fluid Intelligence build setup, not the OSS build path

Closes #471

Verification

• git diff --check
• reviewed the README diff

[grohith327]

@Bortlesboat Can you update the shell script to skip Fluid Intelligence and attempt to build, you can add the instruction back into the README

[Bortlesboat]

Updated in aa66718. The top-level ./build.sh now defaults to the public OSS build, skips the private Fluid Intelligence path, and invokes the unsigned macOS xcodebuild command. ./build.sh fi still preserves the private FI build route. I also added the ./build.sh instruction back to the README.

Verified with bash -n build.sh and a fake xcodebuild shim to confirm the public path reaches the build command. I can't run a real Xcode build from this Windows/WSL environment.