Add missing changes from feature branch

- Update Makefile to run lint and vet before tests
- Add install target to Makefile
- Fix keychainunlocker_stub.go for non-Darwin platforms

internal/secret/keychainunlocker_stub.go

@@ -4,9 +4,8 @@
 package secret
 
 import (
-	"fmt"
-
 	"filippo.io/age"
+	"github.com/awnumar/memguard"
 	"github.com/spf13/afero"
 )
 
@@ -67,3 +66,8 @@ func NewKeychainUnlocker(fs afero.Fs, directory string, metadata UnlockerMetadat
 func CreateKeychainUnlocker(fs afero.Fs, stateDir string) (*KeychainUnlocker, error) {
 	panic("keychain unlockers are only supported on macOS")
 }
+
+// getLongTermPrivateKey panics on non-Darwin platforms
+func getLongTermPrivateKey(fs afero.Fs, vault VaultInterface) (*memguard.LockedBuffer, error) {
+	panic("keychain unlockers are only supported on macOS")
+}