fix(ai-config): hook gptel-magit wiring per-feature, not on magit

The wiring keyed on `with-eval-after-load 'magit` fires while two
of its three references are still undefined.  `magit.el` calls
`(provide 'magit)` BEFORE its `cl-eval-when (load eval)` block
requires `magit-commit` and `magit-stash`.  At that moment the
`magit-commit` transient prefix doesn't exist, and
`transient-append-suffix` silently no-ops on missing prefixes
(default `transient-error-on-insert-failure` is nil).  The "g
Generate commit" and "x Explain" suffixes never landed.  Only the
M-g binding worked, because `git-commit` IS required before
provide.

Three per-feature hooks replace the single `'magit` hook: one each
on `git-commit`, `magit-commit`, and `magit-diff`.  Each hooks the
exact dependency the wiring needs, side-stepping the load-order
race entirely.

The companion test was rewritten to check `after-load-alist`
registration rather than drive the hooks through `provide`.  Emacs
30 batch mode doesn't fire registered `eval-after-load` callbacks
on `provide` alone -- only an actual `load` does.  Inspecting the
registration is the stronger guard anyway: the regression is "a
single `'magit` hook," and the right shape of that check is "no
entry under `magit`, entries under `git-commit`, `magit-commit`,
`magit-diff`."

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