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. Left for later: replacing
an unlocker (#71) and deleting
what an interrupted command leaves under a `.tmp-` name
(#75).

Model: opus-5-5

internal/vault/errors.go

@@ -62,4 +62,10 @@ var (
 	// ErrUnlockerNotFound indicates no unlocker with the given ID exists.
 	// Composed as "unlocker with ID <id> not found".
 	ErrUnlockerNotFound = errors.New("not found")
+
+	// ErrNoLockForFilesystem indicates LockStateDir was given a filesystem
+	// it cannot lock. Composed as "cannot lock the state directory on
+	// filesystem <type>".
+	ErrNoLockForFilesystem = errors.New(
+		"cannot lock the state directory on filesystem")
 )