Fixes the 65 issues reported by the lint job on PR #95 (linux), plus the
issues that surface on the windows lint matrix leg:

- revive exported: add doc comments to exported consts, vars, types,
  functions and methods across the 1Password backends, prompt.go,
  array.go, winhello and cmd/keyring.
- revive unused-parameter: rename unused key params to _.
- staticcheck ST1005: lowercase error strings (sentinel error message
  text changes, e.g. ErrKeyNotFound).
- staticcheck SA1019: drop deprecated rand.Seed in keyctl_test.go.
- noctx: use exec.CommandContext in pass.go, passage.go and tests.
- prealloc: preallocate slices in array.go and test mocks.
- thelper: add t.Helper() to test mock constructors and helpers.
- errname: rename elementNotFoundError -> errElementNotFound (wincred).
- rename tilde_test_linux.go -> tilde_linux_test.go: the old name does
  not end in _test.go, so these tests were compiled into the package but
  never run; they now execute (and pass).
- .golangci.yaml: exclude nilnil in tests, where mocks intentionally
  return nil, nil.
- SA1019: replace deprecated golang.org/x/crypto/ssh/terminal with
  golang.org/x/term (drops the x/crypto dependency entirely)
- ineffassign: drop the unused passphrase assignment from setupTouchID,
  which unlocks the keychain itself; scope passphrase to the else branch
- QF1008: use promoted md.Label instead of md.Item.Label