bugfix(prompting): shlex-parse $EDITOR + thread input_fn through editor fallback

[CryptoJones]

Two related bugs in prompting.py:

1. editor_command() did cmd.split() on the env value, which doesn't
   honor shell quoting. Concrete failure:

   EDITOR="emacsclient -a 'emacs'" -> ['emacsclient', '-a', "'emacs'"]

   The stray quotes get passed to subprocess.run as a literal,
   emacsclient is invoked with an alternate that doesn't exist, fails
   to find a fallback editor, and the multiline answer is lost.

   Fix: shlex.split() honors POSIX-style quoting. Falls back to naive
   split if the user's EDITOR has unbalanced quotes (better than
   returning None silently).

2. _ask_multiline_editor's "no editor found" fallback path called
   _ask_multiline(input, output_fn, q) — using the BUILTIN input
   instead of the input_fn parameter. That bypassed test mocks AND,
   on real runs, meant the operator's input was read from raw stdin
   even when the caller had wired up a special handler (REPL, pipe, etc.).

   Fix: thread input_fn through _ask_multiline_editor as a kwarg with
   default=input, and pass it down to both the fallback path and the
   "required field, re-open editor" recursion.

   Public signature of ask() unchanged; just an extra plumbing argument
   on a private function.

Tests added (3):

• EDITOR="emacsclient -a 'emacs'" -> ['emacsclient', '-a', 'emacs']
• EDITOR with double-quoted Windows-style path with spaces survives
• EDITOR with unbalanced quotes doesn't crash, falls back to naive split

150/150 tests pass; ruff + mypy clean.