ci: Sequence release publication and PR generation (backport #350)

[scouten-adobe]

Hand-resolved backport of #350 to stable. The automated backport (run 36186036296) could not apply it:

CONFLICT (modify/delete): .github/workflows/release-train-cut.yml
  deleted in HEAD and modified in 3f0165c

release-train-cut.yml is a main-only workflow, so the one-line hunk touching it was dropped. Everything else applied cleanly: after this change release.yml, promote.yml, and docs/release-process.md are byte-identical to main, and release-pr.yml is gone from this line too.

What this brings to stable

• Deletes .github/workflows/release-pr.yml. Its independent push/workflow_dispatch trigger is what produced the bogus release PR chore: release v0.29.0-dev #349 against main, and it also raced the publish workflow.
• Moves release-PR generation into a second job in release.yml gated on needs: release-plz, with a fresh fetch-depth: 0 checkout so release-plz sees the tag the publish job just created. Without that ordering, release-plz baselines on the previous tag and proposes a wrong version with the prior release's changelog (the failure seen in chore: release v0.92.0 c2pa-rs#2694, fixed by ci: Sequence release-plz publish and PR jobs c2pa-rs#2698).
• Scopes stale-branch cleanup to release-plz-* branches of closed PRs whose base is the current release line, instead of pruning every release-plz-* branch repo-wide (which could delete another line's in-flight release PR).
• Updates promote.yml narration and docs/release-process.md to match.

Validation

actionlint clean; diff verified identical to main for all three shared files. No remaining references to release-pr.yml.

Note for reviewers

This needs the upstream-first-verified label: dropping the release-train-cut.yml hunk changes the patch id, so upstream-first-check.yml will not match it against main even though the change originated there.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.58%. Comparing base (bc8c3b7) to head (a8a3e4c).

Additional details and impacted files

@@           Coverage Diff           @@
##           stable     #353   +/-   ##
=======================================
  Coverage   72.58%   72.58%           
=======================================
  Files           4        4           
  Lines        1047     1047           
=======================================
  Hits          760      760           
  Misses        287      287           

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