README says secret add drops one trailing newline and import keeps it (closes #158)

`secret add` drops one trailing newline from stdin, so
`echo value | secret add name` stores `value`. `secret import` stores a
file's bytes unchanged, and `secret get` writes the value with no
newline added. The README said none of this, so a PEM key piped into
`add` came back one byte short with no hint why. Its entries for `add`,
`get` and `import` now state each rule. A new test stores `a\n\n`
through `add` and through `import` and checks that `add` kept two bytes
and `import` three. No program code changes.

The new test passes on the code before this change; it fails when `add` keeps the newline.

Model: opus-5-5

internal/cli/trailing_newline_test.go

@@ -0,0 +1,47 @@
+//nolint:testpackage // white-box test of unexported internals
+package cli
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/spf13/afero"
+	"github.com/spf13/cobra"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestAddDropsOneTrailingNewlineImportKeepsIt pins the rule the README
+// gives: `secret add` drops one trailing newline from stdin, and
+// `secret import` stores the file's bytes unchanged.
+func TestAddDropsOneTrailingNewlineImportKeepsIt(t *testing.T) {
+	t.Parallel()
+
+	fs, vlt := newSizeTestVault(t)
+	value := []byte("a\n\n")
+
+	cmd := &cobra.Command{}
+	cmd.SetIn(bytes.NewReader(value))
+
+	cli := NewCLIInstanceWithStateDir(fs, testStateDir)
+	cli.cmd = cmd
+
+	require.NoError(t, cli.AddSecret("added", false))
+
+	sourceFile := "/test/value"
+	require.NoError(t, afero.WriteFile(fs, sourceFile, value, 0o600))
+	require.NoError(t, cli.ImportSecret(cmd, "imported", sourceFile, false))
+
+	added, err := vlt.GetSecret("added")
+	require.NoError(t, err)
+
+	defer added.Destroy()
+
+	imported, err := vlt.GetSecret("imported")
+	require.NoError(t, err)
+
+	defer imported.Destroy()
+
+	assert.Equal(t, []byte("a\n"), added.Bytes())
+	assert.Equal(t, []byte("a\n\n"), imported.Bytes())
+}