Exclude generated build output from npm package

[avarayr]

Closes: #769

LONG VERSION

CameraKit ships a dirty android/build folder in npm.
That folder contains old generated C++ files.
When our app builds:

1. yarn installs CameraKit.
2. CameraKit already contains: android/build/generated/...
3. RN 0.83 tries to generate its own files into that same folder.
4. But Gradle sometimes skips the cleanup step because it thinks schema generation is already up-to-date.
5. So RN writes new files on top of old files.
6. One old .cpp remains: (NativeCameraKitSpecJSI-generated.cpp)
7. That old .cpp expects NativeCameraKitModuleCxxSpecJSI
8. RN 0.83’s new header no longer has that class.
9. C++ compile fails.

SHORT VERSION

RNCK ships old generated files.
RN 0.83 generates new files with new conventions.
Old and new files get mixed.
C++ compiler explodes.

[Copilot]

Pull request overview

This PR updates the library’s npm packaging rules to avoid shipping generated build artifacts (notably android/build/generated) that can conflict with the consuming app’s React Native codegen output (e.g., RN 0.83+), causing Android C++ compile failures.

Changes:

• Add a prepack hook to delete local generated build output before packing/publishing.
• Remove top-level build from the published files whitelist and explicitly exclude android/build from the package contents.

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