refactor(ai-config): switch gptel to local fork, drop tab-width advice

I switched the gptel use-package form to `:load-path "~/code/gptel"`
with `:ensure nil` so Emacs loads from the fork instead of the MELPA
release. The fork now carries the narrow `tab-width' copy in
`gptel-org--create-prompt' that karthink redirected the upstream PR
to, which replaces the local `:around' advice on
`gptel--with-buffer-copy-internal' I'd been carrying.

I also dropped the stale test file
`tests/test-ai-config-gptel-prompt-tab-width.el' and the matching
stub in `tests/testutil-ai-config.el'. Both existed only to test the
advice I removed.

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