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.

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