feat(ai-conversations): add cj/gptel-autosave-toggle with [AS] mode-line indicator

`cj/gptel-autosave-enabled` flipped to t inside the save/load entry
points with no way back off short of editing the variable or
clearing the buffer, and no visible indicator that it was on.

Two pieces:

- `cj/gptel-autosave-toggle` flips the buffer-local state in the
  current GPTel buffer.  Bound to `C-; a A` via `cj/ai-keymap`
  (which-key: "toggle autosave").  When autosave is OFF and no
  filepath is configured yet, the command prompts to save the
  conversation first so a save target exists; otherwise it just
  flips the bit.
- `cj/gptel-autosave-mode-line-format` surfaces " [AS]" in the
  mode-line when autosave is on, blank when off.  Installed via a
  `gptel-mode-hook` so every GPTel buffer picks it up.  The install
  helper is idempotent.

6 new tests cover enable/disable paths, the no-filepath prompt path,
the not-a-gptel-buffer error path, the mode-line format evaluation,
and the install idempotence.

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