refactor(sonar): replace if/throw with require/error

Sonar — replace hand-rolled 'throw IllegalArgumentException' / 'throw
IllegalStateException' with the idiomatic 'require { msg }' / 'error(msg)'.
Exception types preserved exactly (require throws IAE, error throws ISE).

cli/src/main/kotlin/com/vitorpamplona/amethyst/cli/secrets/PassphraseProvider.kt

@@ -42,7 +42,7 @@ class PassphraseProvider(
     ): String {
         fileFlag?.let { path ->
             val text = File(path).readText().trimEnd('\n', '\r')
-            if (text.isEmpty()) throw IllegalArgumentException("--passphrase-file $path is empty")
+            require(text.isNotEmpty()) { "--passphrase-file $path is empty" }
             return text
         }
         System.getenv(envName)?.takeIf { it.isNotEmpty() }?.let { return it }
@@ -52,10 +52,10 @@ class PassphraseProvider(
                     "No TTY and no passphrase source: set $envName or pass --passphrase-file PATH.",
                 )
         val first = console.readPassword("$prompt: ").concatToString()
-        if (first.isEmpty()) throw IllegalArgumentException("empty passphrase")
+        require(first.isNotEmpty()) { "empty passphrase" }
         if (confirm) {
             val second = console.readPassword("Confirm passphrase: ").concatToString()
-            if (first != second) throw IllegalArgumentException("passphrases do not match")
+            require(first == second) { "passphrases do not match" }
         }
         return first
     }

cli/src/main/kotlin/com/vitorpamplona/amethyst/cli/secrets/SecretStore.kt

@@ -91,7 +91,7 @@ class SecretStore internal constructor(
                 when (secret.backend) {
                     MacosKeychainBackend.BACKEND_ID -> MacosKeychainBackend
                     SecretServiceBackend.BACKEND_ID -> SecretServiceBackend
-                    else -> throw IllegalStateException("unknown keychain backend: ${secret.backend}")
+                    else -> error("unknown keychain backend: ${secret.backend}")
                 }
             }