ci: prune pr-build/* branches for closed PRs on trunk pushes

[jkmassel]

Summary

• Each PR build force-pushes a pr-build/<n> snapshot branch (ci(ios): publish + prune per-PR XCFramework snapshot branches #495). Nothing prunes them, so they accumulate.
• Adds a Buildkite step on every trunk push that lists pr-build/* refs, queries each PR's state via the GitHub API, and deletes the refs whose PR is closed (covers both merged and rejected — GitHub collapses them).
• Mirrors Automattic/wordpress-rs#1321, which paired with the equivalent publish PR there.

Changes

• .buildkite/cleanup-pr-build-branches.sh (new): Guards on BUILDKITE_BRANCH == "trunk", sources use-bot-for-git, enumerates via git ls-remote --heads origin 'refs/heads/pr-build/*', queries GET /repos/wordpress-mobile/GutenbergKit/pulls/<n> with GITHUB_TOKEN, and deletes closed-PR refs with batched git push origin :refs/heads/... (chunks of 50). Skips on non-200 responses so we never delete a ref we couldn't verify.
• .buildkite/pipeline.yml: New step gated to build.branch == "trunk", sharing the standard *plugins anchor (CI toolkit + NVM).

Test plan

• First trunk merge after this lands sweeps any orphan pr-build/* branches accumulated from ci(ios): publish + prune per-PR XCFramework snapshot branches #495 testing.
• Subsequent trunk merges delete the just-merged PR's pr-build/<n> branch.
• Step is skipped on PR builds.
• Open-PR pr-build/* branches are left alone.
• If GitHub returns non-200 for a PR (deleted, rate-limited, transient), the corresponding branch is preserved and the step continues.

Related

• Depends on ci(ios): publish + prune per-PR XCFramework snapshot branches #495 to define what pr-build/* branches exist in the first place.

[jkmassel]

Folded into #495 — keeping it as a single PR per @jkmassel.