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/generate.go

@@ -155,6 +155,12 @@ func (cli *Instance) GenerateSecret(
 		return fmt.Errorf("failed to generate random secret: %w", err)
 	}
 
+	release, err := vault.LockStateDir(cli.fs, cli.stateDir)
+	if err != nil {
+		return err
+	}
+	defer release()
+
 	// Store the secret in the vault
 	vlt, err := vault.GetCurrentVault(cli.fs, cli.stateDir)
 	if err != nil {