feat(ai-conversations): autosave on a periodic timer

The existing autosave only fired after gptel-send returned, so a
conversation paused mid-thought wasn't on disk if Emacs crashed. I
added a buffer-local repeating timer that calls
cj/gptel--save-buffer-to-file every cj/gptel-autosave-interval seconds
(default 60) for as long as cj/gptel--autosave-active-p holds.
Toggle-off and kill-buffer-hook cancel it cleanly.

Tests cover start/stop idempotency, the active-p predicate, the
kill-buffer cleanup hook, and the toggle integration.

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