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/cli/lock_test.go

@@ -446,8 +446,6 @@ func TestChangingCommandsWaitForLock(t *testing.T) {
 // state directory lock by the time it writes its output. Holding it while
 // streaming would stall every other changing command for as long as the
 // stream lasts, and forever when the other end of the pipe is one of them.
-//
-//nolint:paralleltest // t.Setenv forbids t.Parallel
 func TestEncryptStreamsUnlocked(t *testing.T) {
 	t.Setenv(secret.EnvMnemonic, testMnemonic)
 
@@ -484,6 +482,7 @@ func TestEncryptStreamsUnlocked(t *testing.T) {
 		// Let encrypt finish, so that it releases the lock, then free it
 		// again for the tests that follow
 		_, _ = io.Copy(io.Discard, outputReader)
+
 		(<-taken)()
 		t.Fatal("secret encrypt held the lock while streaming")
 	}

internal/secret/atomic_test.go

@@ -36,6 +36,8 @@ const currentFile = "current"
 const unlockerMetadataFile = "unlocker-metadata.json"
 
 // unlockerPassphrase protects the passphrase unlockers the tests create.
+//
+//nolint:gosec // G101: test data, not a real credential
 const unlockerPassphrase = "unlocker passphrase"
 
 // hookFs passes every call through to Fs, but first calls before for each
@@ -612,8 +614,6 @@ func TestWriteFileAtomicTempFile(t *testing.T) {
 // vault whose long-term key cannot be had: it must fail without writing
 // anything, so that it never leaves a partial unlocker, nor breaks the one
 // it would replace.
-//
-//nolint:paralleltest // t.Setenv forbids t.Parallel
 func TestPassphraseUnlockerGetsKeyFirst(t *testing.T) {
 	// No mnemonic, and no current unlocker to get the key from
 	t.Setenv(secret.EnvMnemonic, "")
@@ -640,8 +640,6 @@ func TestPassphraseUnlockerGetsKeyFirst(t *testing.T) {
 // passphrase unlocker writes in its directory is its metadata: an unlocker
 // directory without metadata is never used, so one interrupted earlier
 // cannot be.
-//
-//nolint:paralleltest // t.Setenv forbids t.Parallel
 func TestPassphraseUnlockerWritesMetadataLast(t *testing.T) {
 	t.Setenv(secret.EnvMnemonic, testMnemonic)