fix(macos): build native agent for macOS 14+

[TaurusWood]

Summary

Fix the macOS native agent compatibility issue where the distributed binary could be built with the host macOS deployment target instead of the intended macOS 14 minimum target.

The previous build script invoked swiftc directly and bypassed the .macOS(.v14) deployment target declared in Package.swift, which could produce a binary requiring newer macOS runtime libraries.

Changes

• Build the macOS native agent through SwiftPM so Package.swift is the single source of truth for the deployment target
• Ensure the generated binary targets macOS 14+
• Validate that the binary deployment target matches LSMinimumSystemVersion
• Add a macOS 14 compatibility smoke test to prevent regressions

Validation

• Verified on a macOS 14.8.7 Apple Silicon GitHub Actions runner
• Verified the generated binary reports minos=14.0
• Verified the binary does not link against the macOS 26-only libswift_DarwinFoundation2.dylib
• Verified the native agent starts successfully on macOS 14

Full ScreenCaptureKit permission and screenshot interaction flows are not exercised in CI.

Fixes #1