Lock the state directory and write vault files atomically (closes #34)

Each command that changes the state directory holds one lock: flock(2)
on `lock` in the state directory, dropped by the kernel if the process
dies, or a process-wide mutex on the in-memory test filesystem. It
covers the state directory, not each vault, because `currentvault`,
`vault create` and cross-vault moves span vaults, and a lock file in a
vault would be deleted by `vault remove` under a waiting command.
Serializing changes across vaults costs a command-line tool nothing.

Files go through `secret.WriteFileAtomic`; versions, new secrets and
cross-vault copies are built in a temporary directory and renamed into
place; removals rename out of the way first.

Model: opus-5-5

internal/cli/crypto.go

@@ -72,10 +72,18 @@ func newDecryptCmd() *cobra.Command {
 
 // resolveEncryptionKey returns a secure buffer holding the age secret key
 // for the named secret, generating and storing a new key if the secret
-// does not exist. The caller must destroy the returned buffer.
+// does not exist. The caller must destroy the returned buffer. It holds the
+// state directory lock itself, so that Encrypt streams its input and output
+// unlocked and cannot block a secret command at the other end of a pipe.
 func (cli *Instance) resolveEncryptionKey(
 	vlt *vault.Vault, secretName string,
 ) (*memguard.LockedBuffer, error) {
+	release, err := vault.LockStateDir(cli.fs, cli.stateDir)
+	if err != nil {
+		return nil, err
+	}
+	defer release()
+
 	// Check if secret exists
 	secretObj := secret.NewSecret(vlt, secretName)