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).

docs/DESIGN.md

@@ -114,6 +114,18 @@ prints an error, runs nothing of that line and re-prompts. EOF on stdin aborts w
 upgrading (exit 2) so a piped run can never upgrade by accident unless it sends an empty
 line. When purges leave no row at all, the session ends ("Nothing left to upgrade.", exit 0).
 
+Line input. When stdin is a terminal every line is read with bash's readline,
+`IFS= read -e -r -p PROMPT`, with readline's defaults and nothing more: cursor movement and editing
+(Left/Right, Home/End, Ctrl-A/E, Backspace/Delete) and Up/Down over an in-memory history of this
+session's command lines. A line with a non-blank character is added with `history -s` (empty
+lines, the proceed command, are not); Down past the newest entry restores the line typed before Up
+(readline's saved line); Up at the oldest entry and Down on the new line ring the bell. The `Nx`
+`y/N` answer is read the same way and never added. Nothing persists: `HISTFILE` is unset at
+startup and `history -r`/`-w`/`-a` are never used (a non-interactive bash neither loads nor saves
+history by itself). readline draws the prompt and the line on stderr, as bash does. Any other
+stdin keeps the plain `IFS= read -r` with the prompt on stdout and each line echoed after it, so
+piped runs and their logs are unchanged.
+
 Flags shown after a row: `[held]` = the package manager will not upgrade it on its own: on apt
 a hold (`apt-mark hold`) or a package apt keeps back (the "kept back" block of
 `apt-get -s --with-new-pkgs upgrade`), on pacman an `IgnorePkg` (`[ignored]` in `pacman -Qu`).