Keep command output to the README's stdout and stderr rules (closes #224)

The startup banner moves from stdout to stderr, so a `completion`
script, a `config get` value and the hidden `__complete` command print
only their own output. `--quiet`, `--cron` and `--json` still suppress
it.

A failing `remote info`, `prune` or `snapshot remove` under `--json`
now reports its error on stderr. Their reporters returned early under
`--json`, so the failure reached neither stream.

`snapshot verify --quiet` writes no report. A failure is still
returned and printed on stderr, with the same exit status.

Judgement call: the banner's stream, posted on the issue for the owner.

Model: opus-5-5

internal/vaultik/verify_quiet_test.go

@@ -0,0 +1,102 @@
+package vaultik_test
+
+import (
+	"bytes"
+	"context"
+	"io"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/spf13/afero"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/vaultik/internal/log"
+	"sneak.berlin/go/vaultik/internal/snapshot"
+	"sneak.berlin/go/vaultik/internal/ui"
+	"sneak.berlin/go/vaultik/internal/vaultik"
+)
+
+// TestVerify_QuietSuppressesReport is the --quiet contract for
+// `snapshot verify`: neither shallow nor deep verify writes its report,
+// a failed verify still returns its error (which the cli layer prints
+// on stderr), and the --json document still emits.
+func TestVerify_QuietSuppressesReport(t *testing.T) {
+	log.Initialize(log.Config{})
+	t.Parallel()
+
+	fs := afero.NewOsFs()
+	tempDir := t.TempDir()
+
+	dataDir := filepath.Join(tempDir, "source")
+	storeDir := filepath.Join(tempDir, "remote")
+	dbPath := filepath.Join(tempDir, "index.sqlite")
+
+	chunkSize := int64(32 * 1024)
+	maxBlobSize := int64(128 * 1024)
+
+	require.NoError(t, fs.MkdirAll(dataDir, 0o755))
+	require.NoError(t, afero.WriteFile(fs,
+		filepath.Join(dataDir, "data.bin"),
+		bytesPattern("quiet-", int(maxBlobSize*2)), 0o644))
+
+	ctx := context.Background()
+
+	cfg, storer, snapshotID := runFileStorageBackup(
+		ctx, t, fs, dataDir, storeDir, dbPath, chunkSize, maxBlobSize)
+
+	// The UI writes to the same buffer as Stdout, as both write to the
+	// process's stdout in production.
+	var stdout bytes.Buffer
+
+	newQuietVerifier := func() *vaultik.Vaultik {
+		v := &vaultik.Vaultik{
+			Config:  cfg,
+			Storage: storer,
+			Fs:      fs,
+			Stdout:  &stdout,
+			Stderr:  io.Discard,
+			UI:      ui.NewWithColor(&stdout, false),
+		}
+		v.SetContext(ctx)
+		v.UI.SetQuiet(true)
+
+		return v
+	}
+
+	require.NoError(t, newQuietVerifier().VerifySnapshotWithOptions(
+		snapshotID, &vaultik.VerifyOptions{}))
+	require.Empty(t, stdout.String(),
+		"shallow verify must write no report under --quiet")
+
+	require.NoError(t, newQuietVerifier().VerifySnapshotWithOptions(
+		snapshotID, &vaultik.VerifyOptions{Deep: true}))
+	require.Empty(t, stdout.String(),
+		"deep verify must write no report under --quiet")
+
+	require.NoError(t, newQuietVerifier().VerifySnapshotWithOptions(
+		snapshotID, &vaultik.VerifyOptions{JSON: true}))
+	require.Equal(t, "ok", decodeVerifyResult(t, stdout.Bytes()).Status,
+		"the --json document must still emit under --quiet")
+
+	// A snapshot without its encrypted database fails shallow verify. A
+	// failed report also lists each missing blob and each blob of the
+	// wrong size, so remove one blob and grow another.
+	require.NoError(t, os.Remove(filepath.Join(storeDir, "metadata",
+		snapshot.RemoteSnapshotKey(snapshotID), "db.zst.age")))
+
+	blobFiles, err := filepath.Glob(
+		filepath.Join(storeDir, "blobs", "*", "*", "*"))
+	require.NoError(t, err)
+	require.GreaterOrEqual(t, len(blobFiles), 2,
+		"the snapshot must span two blobs, one to remove and one to grow")
+	require.NoError(t, os.Remove(blobFiles[0]))
+	growOneBlob(t, fs, filepath.Join(storeDir, "blobs"))
+
+	stdout.Reset()
+
+	require.Error(t, newQuietVerifier().VerifySnapshotWithOptions(
+		snapshotID, &vaultik.VerifyOptions{}),
+		"--quiet must not change the outcome of a failed verify")
+	require.Empty(t, stdout.String(),
+		"a failed verify must write no report under --quiet")
+}