fix: macOS 13 compatibility (Swift 5.8 + vendored Commander)Macos13 compat

[PrentissLiu]

Summary

This PR brings macOS 13 (Ventura) compatibility by aligning the toolchain and dependencies
with Swift 5.8 / Xcode 14.3.1.

Key changes:

• Lower minimum platform to macOS 13.
• Downgrade swift-tools-version to 5.8.
• Pin SQLite.swift to 0.15.0 (5.7 tools) for Swift 5.8 compatibility.
• Vendor Commander locally and adjust its Package.swift for Swift 5.8.
• Migrate tests from the Swift Testing framework to XCTest.

Rationale

Xcode 14.3.1 on macOS 13 ships Swift 5.8, which cannot parse Swift 6 package manifests.
Commander’s manifest now targets Swift 6, so vendoring a 0.2.0 snapshot (with 5.8-compatible
manifest) is required to build on macOS 13.

Testing

• xcodebuild test -scheme imsg -destination 'platform=macOS' (macOS 13, Xcode 14.3.1) ✅
• make build ✅

Note: swift test via SwiftPM currently crashes (signal 11) on this toolchain, so XCTest
was run through Xcode’s test runner.

Additional Notes

Vendoring Commander keeps the dependency stable for older toolchains. If maintainers prefer
a different approach (e.g., a fork or separate branch), I’m happy to adjust.

[pgYou]

For personal use cases, I've forked this repository and adapted it for macOS 12 with the help of an AI coding tool. Those in need can use it cautiously.
https://github.com/pgYou/imsg/releases/tag/v0.4.1