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.

1 files changed, 30 insertions, 8 deletions

diff --git a/todo.org b/todo.org
index 90017155..c59b41ab 100644
--- a/todo.org
+++ b/todo.org
@@ -2687,14 +2687,36 @@ Survey what published gptel community tools exist (the gptel README, karthink's
 
 Output: a shortlist in =docs/design/gptel-tools-shortlist.org= with the adopt/skip/defer decisions and a follow-up extraction sub-task per "adopt".
 
-*** TODO [#C] Promote gptel-rewrite ergonomics :feature:
-
-=gptel-rewrite= is the killer feature for the keep-gptel decision. Currently bound only to =C-; a r=. Make it land closer to the editing flow:
-
-- A directive-picker wrapper =cj/gptel-rewrite-with-directive= that =completing-read='s a directive name (=terse=, =fix-grammar=, =refactor-readability=, =add-docstring=, =explain-as-comment=, =shorten=) before delegating to =gptel-rewrite=. Directive bodies live in =ai-prompts/= so they share storage with =gptel-prompts=.
-- A =cj/gptel-rewrite-redo-with-different-directive= command for "the last rewrite was close — try this style instead". Walks back to the prior region (or kept selection) and re-prompts.
-
-Open question: should this build on =gptel-rewrite= directly via =:after= advice + state capture, or wrap the call in a =cj/= command that sets =gptel-directives= for the duration?
+*** 2026-05-16 Sat @ 01:54:34 -0500 Added directive-picker wrappers around gptel-rewrite
+
+New module =modules/ai-rewrite.el= with two commands:
+
+- =cj/gptel-rewrite-with-directive= (=C-; a r=, replacing the 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 (markers are saved
+  buffer-local so the region survives accept/reject of the first
+  rewrite).
+
+Open-question answer: the directive is injected via a one-shot
+=let=-binding on =gptel-rewrite-directives-hook= (an abnormal hook
+that gptel-rewrite already supports for per-call system messages),
+not by mutating =gptel-directives= globally.  No advice on
+=gptel-rewrite= and no state to clean up after the call returns.
+
+Directives ship inline as a =defcustom= alist with the six names
+called out in the task body (=terse=, =fix-grammar=,
+=refactor-readability=, =add-docstring=, =explain-as-comment=,
+=shorten=) so customization is straightforward without a separate
+file layer.
+
+9 tests in =tests/test-ai-rewrite.el= cover the defcustom shape,
+the wrapper (normal path, no-region error, unknown-directive
+error, last-state recording), and the redo (replays prior region,
+errors when no previous, excludes the current directive from the
+re-pick prompt).  =gptel-rewrite= stubbed for tests so no rewrite
+UI fires.
 
 *** TODO [#B] Saved-conversations browser :feature: