Restore point when no prompt is found

[OSadovy]

The problem

I use agent-shell. Claude was in plan mode: it wrote a long plan into the shell
buffer and stopped for approval. The plan was plainly there in the shell buffer,
but agent-shell's viewport - which re-derives the current interaction through
shell-maker--command-and-response-at-point - showed the response cut off long
before the plan, ending part-way through an earlier tool call.

Earlier in that session the agent had read agent-shell's own source into the
transcript, so the buffer contained the literal text :shell-prompt "Claude> ".
That is what triggered it, but the lookalike text isn't the bug:
shell-maker--re-search-forward-prompt correctly rejected all four occurrences.

Cause

shell-maker--re-search-forward-prompt loops re-search-forward, moving point
onto each candidate and rejecting those without the comint-highlight-prompt
face. When it runs out of candidates it returns nil - but point is left on the
last rejected match instead of where it started.

shell-maker-narrow-to-prompt discards the return value and reads point instead:

(goto-char (shell-maker--prompt-end-position))
(shell-maker--re-search-forward-prompt ...)
(if (= begin (shell-maker--prompt-begin-position))
    (point-max)
  (shell-maker--prompt-begin-position))

"Did I move?" is a reasonable way to ask whether there was a next prompt, but it
only holds if a failed search leaves point alone. In my buffer point had been
dragged ~240k characters forward, so --prompt-begin-position answered with a
lookalike and narrowing stopped there. Everything past it - the entire plan -
fell outside the interaction.

The trailing prompt is why this isn't seen more often: after a finished exchange
there is a real prompt at the end, the search finds it, and nothing leaks. It
only goes wrong while the shell is still busy - which is exactly when a plan is
on screen waiting to be answered.

Fix

Restore point when nothing is found, the way re-search-forward itself does.

The other two callers, shell-maker-history-position and
shell-maker--extract-history, run inside their own save-excursion and don't
read point after a nil return, so they are unaffected.

Reproducing

Anchored regexp, no agent-shell, no real session data:

(require 'shell-maker)

(let* ((config (make-shell-maker-config
                :name "Repro"
                :prompt "Agent> "
                :prompt-regexp "^Agent> "
                ;; Write output but never finish, so no new prompt is
                ;; printed: the shell is still working.
                :execute-command
                (lambda (_command shell)
                  (funcall (map-elt shell :write-output)
                           (concat "Here is a transcript excerpt:\n"
                                   "Agent> hello\n"
                                   "and THE-TAIL of the answer\n"))))))
  (shell-maker-start-v2 :config config :no-focus t)
  (with-current-buffer (shell-maker-buffer config)
    (goto-char (point-max))
    (insert "explain")
    (shell-maker-submit)
    (goto-char comint-last-input-start)
    (princ (format "%S\n" (cdr (shell-maker--command-and-response-at-point))))))

Before: "Here is a transcript excerpt:" - the response stops at the lookalike.
After: the full three lines.

Note the regexp is anchored, so this is not about unanchored prompt regexps.
(agent-shell does pass unanchored ones)

Test

shell-maker-history-test-search-prompt-leaves-point-when-not-found, added next
to the existing -extract-preserves-point. Fails before the change, passes
after, and the other 14 in that file still pass.

What else I considered

shell-maker--prompt-begin-position searches backward with a plain
re-search-backward comint-prompt-regexp and no face check, so it can still
anchor on a lookalike sitting between the real prompt and point. Guarding it
would independently have saved me here, but it decides where interactions begin
for shell-maker-delete-interaction-at-point too, and it needs a "found
nothing" case designed. I'd rather get package author's call on it.