feat(prompt): readline line editing with per-session history

When stdin is a terminal the prompt is read with bash's readline,
`IFS= read -e -r -p '> ' REPLY`, so the arrow keys no longer print escape
garbage: Left/Right, Home/End, Ctrl-A/E, Backspace/Delete edit the line,
and Up/Down walk the lines entered in this session (`history -s` for every
line with a non-blank character; empty lines are not recorded). Down past
the newest entry restores the line typed before Up, and Up at the oldest
entry or Down on the new line ring the bell, as readline does by default.
The Nx y/N answer is read the same way and never recorded.

Nothing persists: HISTFILE is unset at startup and the history is never
read or written. Verified on bash 4.3.48 and 5.2.15 that `read -e` and
`history -s` share one history list in a non-interactive shell, that
inherited HISTSIZE/HISTCONTROL/HISTIGNORE have no effect there, and that
Ctrl-C at the prompt restores the terminal. readline draws the prompt on
stderr, as bash does. Piped stdin keeps the plain `read -r` exactly,
including the echo of each line and EOF => exit 2.

New unit tests (tests/unit/lineedit.bats) drive a real pty with script(1):
history recall, the bell at both ends, the stashed line, Left-arrow
editing, the y/N answer kept out of the history, no history file under
HOME/TMPDIR, ^D => exit 2. The E2E suite gets a pty smoke test (Up
recalls 1?), skipped where the image has no script(1).

tests/unit/test_helper.bash

@@ -50,6 +50,28 @@ use_fakes() {
 # skup ARGS...: run the applet (use as `run skup ...`, stdin is passed through)
 skup() { env PATH="$TEST_PATH" "$SU_BIN" "$@"; }
 
+# pty_skup INPUT ARGS...: run the applet on a pseudo-terminal (script(1), util-linux/bsdutils) with
+# INPUT typed in one go - readline consumes it as it reads, so no timing is involved; end INPUT with
+# a line that ends the session (q or an empty line). Sets status, output and lines like `run`
+# (output without the pty's CRs; NO_COLOR, as stdout is a terminal); the raw transcript stays in
+# $PTY_RAW. Skips without script(1).
+# shellcheck disable=SC2034 # status/lines are read by bats-assert
+pty_skup() {
+	local input=$1 cmd
+	shift
+	command -v script >/dev/null 2>&1 || skip "script(1) (util-linux/bsdutils) not available: no pty to drive"
+	PTY_RAW=$BATS_TEST_TMPDIR/pty.raw
+	printf -v cmd '%q ' env PATH="$TEST_PATH" TERM=dumb NO_COLOR=1 "$SU_BIN" "$@"
+	status=0
+	printf '%s' "$input" | timeout 60 script -qfec "$cmd" /dev/null >"$PTY_RAW" 2>&1 || status=$?
+	output=$(tr -d '\r' <"$PTY_RAW")
+	mapfile -t lines <<<"$output"
+}
+# pty_bells: number of BEL bytes (readline's bell) in the raw transcript of the last pty_skup
+pty_bells() { tr -cd '\a' <"$PTY_RAW" | wc -c; }
+# count_lines LINE: number of output lines that are exactly LINE
+count_lines() { grep -cxF -- "$1" <<<"$output" || true; }
+
 # shellcheck disable=SC2034 # used by the .bats files
 HEADER='# skeptical-updater trusted packages. Columns: backend  package  reason  added(UTC ISO-8601)'
 # shellcheck disable=SC2034