WIP: refactor to use memguard for secure memory handling

- Add memguard dependency
- Update ReadPassphrase to return LockedBuffer
- Update EncryptWithPassphrase/DecryptWithPassphrase to accept LockedBuffer
- Remove string wrapper functions
- Update all callers to create LockedBuffers at entry points
- Update interfaces and mock implementations

go.mod

@@ -4,6 +4,7 @@ go 1.24.1
 
 require (
 	filippo.io/age v1.2.1
+	github.com/awnumar/memguard v0.22.5
 	github.com/btcsuite/btcd v0.24.2
 	github.com/btcsuite/btcd/btcec/v2 v2.1.3
 	github.com/btcsuite/btcd/btcutil v1.1.6
@@ -18,6 +19,7 @@ require (
 )
 
 require (
+	github.com/awnumar/memcall v0.2.0 // indirect
 	github.com/btcsuite/btcd/chaincfg/chainhash v1.1.0 // indirect
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/decred/dcrd/dcrec/secp256k1/v4 v4.0.1 // indirect