feat: undo last rating, customizable keys, and configurable text limit

A batch of self-contained user-facing improvements, squashed from the feat/org-drill-solo-features branch.

I added an undo for the last rating (issue #2 follow-up). The rating prompt now takes an undo key (org-drill--undo-key, default u): it restores the previous card's scheduling snapshot, drops the recorded quality, and re-queues that card, then returns to the current prompt. Each rating snapshots the scheduling properties and SCHEDULED line onto a per-session stack capped at org-drill-undo-limit (default 3). org-drill-reschedule loops on the rating read so undo doesn't rate the current card.

I made the five session-control keys (quit, edit, help, skip, tags) defcustoms so they can be rebound from customize-group (issue #35), keeping their defaults. The 0-5 rating keys stay as-is, since they're tied to the quality scale rather than being variables.

I lifted the hardcoded 100-line entry-text limit in org-drill-get-entry-text into the org-drill-entry-text-max-lines defcustom, defaulting to 100.

I also deleted a commented-out old org-entry-empty-p that the real definition had already replaced.

Existing tests stay green and each change added its own, including snapshot/restore and prompt-loop tests for undo.

1 files changed, 33 insertions, 0 deletions

diff --git a/tests/test-org-drill-session-keys.el b/tests/test-org-drill-session-keys.el
new file mode 100644
index 0000000..ff1b4b1
--- /dev/null
+++ b/tests/test-org-drill-session-keys.el
@@ -0,0 +1,33 @@
+;;; test-org-drill-session-keys.el --- Tests for customizable session keys  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; The session-control keys (quit/edit/help/skip/tags) are defcustoms so they
+;; can be rebound through customize-group (upstream issue #35), while keeping
+;; their historical default characters.
+
+;;; Code:
+
+(require 'ert)
+(require 'org-drill)
+
+(defconst test-org-drill-session-key-defaults
+  '((org-drill--quit-key . ?q)
+    (org-drill--edit-key . ?e)
+    (org-drill--help-key . ??)
+    (org-drill--skip-key . ?s)
+    (org-drill--tags-key . ?t))
+  "Each session-control key variable and its historical default character.")
+
+(ert-deftest test-org-drill-session-keys-are-customizable ()
+  "Each session-control key is a defcustom, so customize-group can set it."
+  (dolist (cell test-org-drill-session-key-defaults)
+    (should (custom-variable-p (car cell)))))
+
+(ert-deftest test-org-drill-session-keys-keep-their-defaults ()
+  "Promoting to defcustom does not change the default bindings."
+  (dolist (cell test-org-drill-session-key-defaults)
+    (should (eq (cdr cell) (default-value (car cell))))))
+
+(provide 'test-org-drill-session-keys)
+
+;;; test-org-drill-session-keys.el ends here