fix(ci): trigger release PR auto-merge via workflow_run instead of pull_request - #479

## Problem

Follow-up to #479. Confirmed live right after #479 merged: the new
`find-pr` job ran and correctly located PR #476, but skipped it with
"Open PR doesn't match expected author/label", leaving the release PR
un-merged.

Root cause: `gh pr list --json author` is GraphQL-backed and represents
`github-actions[bot]` as `"app/github-actions"`, not
`"github-actions[bot]"`. The comparison in `find-pr` (and the reusable
workflow's `release_pr_author` default, and the old
`pull_request`-triggered version's
`github.event.pull_request.user.login`) all expect the REST-style
`"github-actions[bot]"`.

## Fix

Switch the PR lookup from `gh pr list --json author` to `gh api
repos/{repo}/pulls?state=open&head={owner}:release-please--branches--main`
(REST), whose `user.login` is `"github-actions[bot]"` as expected.
Verified locally against the real PR #476 before pushing.

## Test plan

- [ ] Merge this PR.
- [ ] Confirm in Actions that the next `auto-merge-release-pr`
(workflow_run) run resolves `pr_number` for PR #476 and actually merges
it this time.
- [ ] Confirm the tag/GitHub Release gets published without human
intervention.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

🤖 I have created a release *beep* *boop*
---


##
[6.10.0](v6.9.0...v6.10.0)
(2026-08-07)


### Features

* **service_definition:** forward link spec fields from the template
([#477](#477))
([2994e6b](2994e6b))


### Bug Fixes

* **ci:** compare release PR author against REST-style bot login
([4e00860](4e00860))
* **ci:** compare release PR author against REST-style bot login
([#481](#481))
([75a16ea](75a16ea))
* **ci:** trigger release PR auto-merge via workflow_run instead of
pull_request
([e7eff48](e7eff48))
* **ci:** trigger release PR auto-merge via workflow_run instead of
pull_request
([#479](#479))
([80738de](80738de))
* **eks:** support an optional aws_profile for the kubernetes provider
exec plugin
([#471](#471))
([ccb2f5f](ccb2f5f))

---
This PR was generated with [Release
Please](https://github.com/googleapis/release-please). See
[documentation](https://github.com/googleapis/release-please#release-please).

Small, low-risk change used as a real trigger to test the full
post-merge automation chain (#479, #481,
nullplatform/actions-nullplatform#89) end-to-end on a clean cycle: this
PR's merge → release-please PR → auto-merge (workflow_run + REST author
check + --admin) → tag/release, with no manual step in between.

Root README's usage example pinned `infrastructure/aws/vpc` to
`v1.56.1`, several major versions behind the current `v6.10.0` — updated
so copy-pasting the example doesn't point at a stale tag.

🤖 Generated with [Claude Code](https://claude.com/claude-code)