Limit model cache cleanup to repo-owned WhisperKit paths

[justingleader]

Summary

• replace broad substring-based cleanup with exact repo-owned WhisperKit cache path resolution
• delete only exact variant directories under the argmaxinc/whisperkit-coreml cache roots
• add tests that cover safe candidate selection and exact-match deletion behavior

Testing

• xcodebuild -project /Users/justinleader/Documents/Codex/2026-04-23-examine-this-fully-and-see-if/speaktype/speaktype.xcodeproj -scheme speaktype build-for-testing -destination 'platform=macOS' MACOSX_DEPLOYMENT_TARGET=15.0 CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO -only-testing:speaktypeTests/ModelDownloadServiceTests\n- xcodebuild -project /Users/justinleader/Documents/Codex/2026-04-23-examine-this-fully-and-see-if/speaktype/speaktype.xcodeproj -scheme speaktype test-without-building -destination 'platform=macOS' MACOSX_DEPLOYMENT_TARGET=15.0 CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO -only-testing:speaktypeTests/ModelDownloadServiceTests