- Add build tags to keychain implementation files (Darwin-only)
- Create stub implementations for non-Darwin platforms that panic
- Conditionally show keychain support in help text based on platform
- Platform check in UnlockersAdd prevents keychain usage on non-Darwin
- Verified GPG operations already protected against command injection
via validateGPGKeyID() and proper exec.Command argument passing
- Keychain operations use go-keychain library, no shell commands
The application now builds and runs on Linux/non-Darwin platforms with
keychain functionality properly isolated to macOS only.
