`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
