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.

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. An unlocker added under
an existing unlocker's directory name is still rewritten file by file:
#71.

Model: opus-5-5

internal/cli/unlockers.go

@@ -534,6 +534,12 @@ func (cli *Instance) printUnlockersTable(unlockers []UnlockerInfo) error {
 
 // UnlockersAdd adds a new unlocker
 func (cli *Instance) UnlockersAdd(unlockerType string, cmd *cobra.Command) error {
+	release, err := vault.LockStateDir(cli.fs, cli.stateDir)
+	if err != nil {
+		return err
+	}
+	defer release()
+
 	switch unlockerType {
 	case unlockerTypePassphrase:
 		return cli.addPassphraseUnlocker(cmd)
@@ -714,6 +720,12 @@ func (cli *Instance) addPGPUnlocker(cmd *cobra.Command) error {
 func (cli *Instance) UnlockersRemove(
 	unlockerID string, force bool, cmd *cobra.Command,
 ) error {
+	release, err := vault.LockStateDir(cli.fs, cli.stateDir)
+	if err != nil {
+		return err
+	}
+	defer release()
+
 	// Get current vault
 	vlt, err := vault.GetCurrentVault(cli.fs, cli.stateDir)
 	if err != nil {
@@ -763,6 +775,12 @@ func (cli *Instance) UnlockersRemove(
 
 // UnlockerSelect selects an unlocker as current
 func (cli *Instance) UnlockerSelect(unlockerID string) error {
+	release, err := vault.LockStateDir(cli.fs, cli.stateDir)
+	if err != nil {
+		return err
+	}
+	defer release()
+
 	// Get current vault
 	vlt, err := vault.GetCurrentVault(cli.fs, cli.stateDir)
 	if err != nil {