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/vault/metadata.go

@@ -113,7 +113,7 @@ func SaveVaultMetadata(fs afero.Fs, vaultDir string, metadata *Metadata) error {
 		return fmt.Errorf("failed to marshal vault metadata: %w", err)
 	}
 
-	err = afero.WriteFile(fs, metadataPath, metadataBytes, secret.FilePerms)
+	err = secret.WriteFileAtomic(fs, metadataPath, metadataBytes)
 	if err != nil {
 		return fmt.Errorf("failed to write vault metadata: %w", err)
 	}