Move the public-surface migration gate to its own pull_request workflow (#287)

The public-surface migration gate needs a pull request's base/head SHAs,
so it can't run on a plain push — it evaluates its `if` to false and
shows up as a **skipped** check on every push, main included. This moves
it out of the shared `[push, pull_request]` workflow into its own `on:
pull_request` workflow, so on push runs the job simply doesn't exist and
no skipped check appears.

On pull requests it runs exactly as before: the bootstrap exemption
(skip when `.public-surface` is first introduced) and the
`[no-migration]` title escape hatch are both intact.

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