ci(pr): classify changed files from the merge ref when the files API 422s

The `changes` job decides whether a PR touches product code, and ci-ok
requires it. It asks GitHub for the PR's file list. An earlier fix moved
it off the .diff endpoint, which rejects PRs over 20k lines, onto the
paginated pulls/files endpoint -- but that endpoint renders the diff
server-side as well, and for a PR that regenerates a vendored parser it
answers

    HTTP 422: Sorry, this diff is taking too long to generate.

Observed on #2246 (a 42 MB sql/parser.c): `changes` red, therefore ci-ok
red, for a reason that has nothing to do with the contribution. The
compare endpoint fails the same way, so there is no API route to the
file list for such a PR. Every grammar refresh would hit this.

When the API call fails, the job now takes the list from git instead.
The PR merge ref's first parent is the base, so a name-only diff across
that one merge commit is exactly the PR's changed files. The fetch is
depth 2 with --filter=blob:none into a bare repository under RUNNER_TEMP:
commits and trees only, no blobs, nothing checked out and nothing from
the PR executed. No new action, no new permission, and the API path and
the classification regex are untouched, so ordinary PRs behave as before.
A ::notice:: line records when the fallback was used. If the fetch fails
too, the step exits non-zero and writes no output -- the gate fails
closed, as it did before.

Proof, by executing the workflow's own run block under
`bash -eo pipefail` with a stub gh:

  API path, docs-only list        rc 0    product=false
  fallback, real #2246            rc 0    product=true, the same five
                                          files `git diff --numstat`
                                          reports for the PR; 0.85 s,
                                          188 KB fetched
  fallback, nonexistent merge ref rc 128  no output written

Contract tests that read pr.yml -- security_gate_fail_closed,
smoke_fixture_contract, windows_bundle_contract, venue_parity_contract --
pass.

Not addressed here, noted for a follow-up: the classification pipes the
list into `grep -q` under pipefail, which can report a match as a
failure once the list outgrows the pipe buffer.

Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>