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/vault/management.go

@@ -169,7 +169,7 @@ func processMnemonicForVault(
 
 	ltPubKeyPath := filepath.Join(vaultDir, "pub.age")
 
-	err = afero.WriteFile(fs, ltPubKeyPath, []byte(ltPubKey), secret.FilePerms)
+	err = secret.WriteFileAtomic(fs, ltPubKeyPath, []byte(ltPubKey))
 	if err != nil {
 		return 0, "", "", fmt.Errorf("failed to write long-term public key: %w", err)
 	}
@@ -295,21 +295,13 @@ func SelectVault(fs afero.Fs, stateDir string, name string) error {
 		return fmt.Errorf("vault %s %w", name, ErrVaultNotFound)
 	}
 
-	// Create or update the currentvault file with just the vault name
+	// Create or replace the currentvault file with just the vault name. It
+	// is replaced in one rename, so it never goes missing.
 	currentVaultPath := filepath.Join(stateDir, "currentvault")
 
-	// Remove existing file if it exists
-	_, err = fs.Stat(currentVaultPath)
-	if err == nil {
-		secret.Debug("Removing existing currentvault file", "path", currentVaultPath)
-
-		_ = fs.Remove(currentVaultPath)
-	}
-
-	// Write just the vault name to the file
 	secret.Debug("Writing currentvault file", "vault_name", name)
 
-	err = afero.WriteFile(fs, currentVaultPath, []byte(name), secret.FilePerms)
+	err = secret.WriteFileAtomic(fs, currentVaultPath, []byte(name))
 	if err != nil {
 		return fmt.Errorf("failed to select vault: %w", err)
 	}