feat(ai-rewrite): add directive-picker wrappers around gptel-rewrite

`gptel-rewrite` is the killer feature for the keep-gptel decision,
and it now lives behind two commands instead of the bare call:

- `cj/gptel-rewrite-with-directive` (`C-; a r`, replacing the
  former bare `gptel-rewrite` binding): completing-read on a
  directive name from `cj/gptel-rewrite-directives`, then rewrite
  the active region.
- `cj/gptel-rewrite-redo-with-different-directive` (`C-; a R`):
  replay the prior region with a different directive.  The region
  is preserved via markers stored buffer-local on the first call so
  it survives accept/reject of the prior rewrite.

I picked the hook injection approach over an `:after`-advice +
state-capture pattern.  `gptel-rewrite-directives-hook` is an
abnormal hook gptel-rewrite already consults for a per-call
system message.  Wrapping the call in a one-shot `let`-binding on
that hook gives the directive exactly the lifetime of the rewrite
and leaves nothing to clean up.  Mutating `gptel-directives`
globally would mean either restoring it afterward or living with
the change -- both worse than the hook.

Directives ship inline as a `defcustom` alist with the six names
called out in the task -- `terse`, `fix-grammar`,
`refactor-readability`, `add-docstring`, `explain-as-comment`,
`shorten`.  Customization is a `customize-variable` or `setq`
away.

9 tests cover the defcustom shape (default names present, bodies
non-empty strings), the wrapper (normal path, no-region error,
unknown-directive error, last-state recording), and the redo
(replays the prior region, errors when no previous, excludes the
current directive from the re-pick prompt).  `gptel-rewrite`
stubbed in tests so no rewrite UI fires.

2 files changed, 114 insertions, 2 deletions

diff --git a/modules/ai-config.el b/modules/ai-config.el
index 6eff1ba6..c7a14cae 100644
--- a/modules/ai-config.el
+++ b/modules/ai-config.el
@@ -35,6 +35,8 @@
 (autoload 'cj/gptel-delete-conversation "ai-conversations" "Delete a saved AI conversation." t)
 (autoload 'cj/gptel-autosave-toggle "ai-conversations" "Toggle autosave in the current GPTel buffer." t)
 (autoload 'cj/gptel-quick-ask "ai-quick-ask" "One-shot quick-ask in a transient buffer." t)
+(autoload 'cj/gptel-rewrite-with-directive "ai-rewrite" "Pick a directive and run gptel-rewrite on the region." t)
+(autoload 'cj/gptel-rewrite-redo-with-different-directive "ai-rewrite" "Re-run the previous rewrite with a different directive." t)
 
 ;;; ------------------------- AI Config Helper Functions ------------------------
 
@@ -510,7 +512,8 @@ Works for any buffer, whether it's visiting a file or not."
   "m" #'cj/gptel-change-model        ;; change the LLM model
   "p" #'gptel-system-prompt          ;; change prompt
   "q" #'cj/gptel-quick-ask           ;; one-shot quick ask
-  "r" #'gptel-rewrite                ;; rewrite a region of code/text
+  "r" #'cj/gptel-rewrite-with-directive ;; rewrite region with a chosen directive
+  "R" #'cj/gptel-rewrite-redo-with-different-directive ;; redo last rewrite, new directive
   "c" #'cj/gptel-context-clear       ;; clear all context
   "s" #'cj/gptel-save-conversation   ;; save conversation
   "t" #'cj/toggle-gptel              ;; toggles the ai-assistant window
@@ -530,7 +533,8 @@ Works for any buffer, whether it's visiting a file or not."
     "C-; a m" "change model"
     "C-; a p" "change prompt"
     "C-; a q" "quick ask"
-    "C-; a r" "rewrite region"
+    "C-; a r" "rewrite region (directive)"
+    "C-; a R" "redo rewrite, new directive"
     "C-; a c" "clear context"
     "C-; a s" "save conversation"
     "C-; a t" "toggle window"
diff --git a/modules/ai-rewrite.el b/modules/ai-rewrite.el
new file mode 100644
index 00000000..fb25c137
--- /dev/null
+++ b/modules/ai-rewrite.el
@@ -0,0 +1,108 @@
+;;; ai-rewrite.el --- Directive-picker wrappers for gptel-rewrite -*- lexical-binding: t; coding: utf-8; -*-
+
+;; Author: Craig Jennings <c@cjennings.net>
+
+;;; Commentary:
+;; Adds two ergonomic wrappers around `gptel-rewrite':
+;;
+;;   cj/gptel-rewrite-with-directive       Pick a named directive,
+;;                                         then rewrite the region.
+;;   cj/gptel-rewrite-redo-with-different-directive
+;;                                         Re-run the previous region
+;;                                         with a different directive.
+;;
+;; A directive is a short system-message snippet attached to a name
+;; (e.g. "terse", "fix-grammar").  The directive body is injected
+;; into the rewrite via `gptel-rewrite-directives-hook' just for that
+;; call -- no global state changes.
+
+;;; Code:
+
+;; Declare the hook variable special so our `let'-binding below is
+;; dynamic (visible across the `call-interactively' that follows)
+;; rather than lexical when this file is byte-compiled.
+(defvar gptel-rewrite-directives-hook)
+
+(defcustom cj/gptel-rewrite-directives
+  '(("terse"
+     . "Rewrite the text to be as terse as possible without losing meaning.\nDo not add commentary.  Return only the rewritten text.")
+    ("fix-grammar"
+     . "Fix grammar and spelling errors only.  Do not rephrase, restructure,\nor change tone.  Return only the corrected text.")
+    ("refactor-readability"
+     . "Refactor the code for readability.  Improve naming, split long\nfunctions when appropriate, remove unnecessary complexity, and preserve\nbehavior exactly.  Return only the refactored code.")
+    ("add-docstring"
+     . "Add or improve docstrings for every function in the region.  Use the\nidiomatic docstring style for the language.  Do not change executable\ncode.  Return the whole region with the updated docstrings.")
+    ("explain-as-comment"
+     . "Replace the region with the original code, preceded by a concise\nblock comment explaining what the code does.  Use the language's\nidiomatic comment syntax.  Return code + comment, nothing else.")
+    ("shorten"
+     . "Shorten the text while preserving meaning, technical accuracy, and\nthe author's voice.  Remove rhetorical padding.  Return only the\nshortened text."))
+  "Named system-message directives for `cj/gptel-rewrite-with-directive'.
+Each entry is a (NAME . BODY) pair where NAME is the directive label
+presented in the completing-read prompt and BODY is the system
+message injected into the next `gptel-rewrite' call."
+  :type '(alist :key-type string :value-type string)
+  :group 'cj)
+
+(defvar-local cj/gptel-rewrite--last-region nil
+  "Cons (BEG-MARKER . END-MARKER) of the last directive-driven rewrite.")
+
+(defvar-local cj/gptel-rewrite--last-directive nil
+  "Name of the directive used in the last directive-driven rewrite.")
+
+(defun cj/gptel-rewrite--call-with-directive (directive-name beg end)
+  "Run `gptel-rewrite' over BEG..END with DIRECTIVE-NAME's system message.
+Stores the region (as markers) and directive name on buffer-local
+variables so `cj/gptel-rewrite-redo-with-different-directive' can
+revisit them."
+  (let ((body (alist-get directive-name cj/gptel-rewrite-directives
+                         nil nil #'equal)))
+    (unless body
+      (user-error "Unknown rewrite directive: %s" directive-name))
+    (setq-local cj/gptel-rewrite--last-region
+                (cons (copy-marker beg) (copy-marker end)))
+    (setq-local cj/gptel-rewrite--last-directive directive-name)
+    (let ((gptel-rewrite-directives-hook
+           (cons (lambda () body) gptel-rewrite-directives-hook)))
+      (save-excursion
+        (goto-char beg)
+        (push-mark end t t)
+        (call-interactively #'gptel-rewrite)))))
+
+;;;###autoload
+(defun cj/gptel-rewrite-with-directive (directive-name)
+  "Pick DIRECTIVE-NAME from `cj/gptel-rewrite-directives' and rewrite the region.
+Requires an active region.  The directive is applied only to this
+call -- it does not modify global `gptel-directives'."
+  (interactive
+   (progn
+     (unless (use-region-p)
+       (user-error "No region selected"))
+     (list (completing-read
+            "Rewrite directive: "
+            (mapcar #'car cj/gptel-rewrite-directives) nil t))))
+  (cj/gptel-rewrite--call-with-directive
+   directive-name (region-beginning) (region-end)))
+
+;;;###autoload
+(defun cj/gptel-rewrite-redo-with-different-directive ()
+  "Re-run the previous directive-driven rewrite with a different directive.
+The region is restored from the markers captured at the last call;
+the user picks a new directive from the remaining choices."
+  (interactive)
+  (unless cj/gptel-rewrite--last-region
+    (user-error "No previous rewrite to redo in this buffer"))
+  (let* ((beg-mk (car cj/gptel-rewrite--last-region))
+         (end-mk (cdr cj/gptel-rewrite--last-region))
+         (current cj/gptel-rewrite--last-directive)
+         (others (cl-remove
+                  current
+                  (mapcar #'car cj/gptel-rewrite-directives)
+                  :test #'equal))
+         (chosen (completing-read
+                  (format "Re-rewrite with (was %s): " current)
+                  others nil t)))
+    (cj/gptel-rewrite--call-with-directive
+     chosen (marker-position beg-mk) (marker-position end-mk))))
+
+(provide 'ai-rewrite)
+;;; ai-rewrite.el ends here