Print usage only for a command called wrongly (closes #41)

A failed command printed the whole usage text after its error, burying
it. The root command's PersistentPreRunE now turns usage off, so an
error from running the command is printed once on its own. Wrong arity,
an unknown flag, a bad flag value, a missing required flag and broken
flag groups still get usage: cobra checks arguments and flag values
before that hook but required flags and flag groups only after it, so
the hook checks those two first. Root SilenceUsage was not used: in
this cobra version it hides usage for argument and flag errors too.
Cobra still prints the error; Entry is unchanged.

Model: opus-5-5

internal/cli/usage_test.go

@@ -0,0 +1,46 @@
+package cli_test
+
+import (
+	"strings"
+	"testing"
+
+	"git.eeqj.de/sneak/secret/internal/cli"
+	"git.eeqj.de/sneak/secret/internal/secret"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// usageHeading starts the usage text cobra prints after an error.
+const usageHeading = "Usage:"
+
+// A command called wrongly gets usage after its error; a command that
+// fails while running gets its error alone. Either way the command fails
+// and its error is shown exactly once.
+//
+//nolint:paralleltest // executes the CLI in-process and sets the environment
+func TestUsageOnlyForCallErrors(t *testing.T) {
+	// No vault in the state directory, so `get x` fails while running.
+	env := map[string]string{secret.EnvStateDir: t.TempDir()}
+
+	tests := []struct {
+		call      string
+		wantUsage bool
+	}{
+		{call: "get", wantUsage: true},
+		{call: "get x y", wantUsage: true},
+		{call: "get --no-such-flag x", wantUsage: true},
+		{call: "generate secret x --length abc", wantUsage: true},
+		{call: "import x", wantUsage: true},
+		{call: "get x", wantUsage: false},
+	}
+
+	for _, tt := range tests {
+		output, err := cli.ExecuteCommandInProcess(strings.Fields(tt.call), "", env)
+		require.Error(t, err, "%q should fail", tt.call)
+
+		assert.Equal(t, 1, strings.Count(output, err.Error()),
+			"%q should show its error once:\n%s", tt.call, output)
+		assert.Equal(t, tt.wantUsage, strings.Contains(output, usageHeading),
+			"usage shown for %q:\n%s", tt.call, output)
+	}
+}