Derive the PR for automations from the triggering run

[obenland]

Description

pr_automations.yml identified the pull request and event from JSON files uploaded by pr_automations_init.yml, and unpacked both artifact archives into its checkout of main. Since the init run executes pull request code, this changes the handoff so the privileged side derives what it can itself:

• The pull request is resolved from the workflow_run payload by head repository and branch, matching the run's head SHA where possible. workflow_run.pull_requests is empty for these runs, so it is not used.
• Only the event action and changed groups are read from the artifacts, by copying the one expected entry out of each archive rather than unpacking it, and both values are validated before use.
• The job gets read-only default permissions, the checkout no longer persists credentials, and the two automation steps only run once the pull request is resolved.

pr_automations_init.yml is unchanged, so runs from branches opened before this lands keep working.

Testing Instructions

Open, edit or review a pull request after this is merged and confirm the "PR automations" run resolves the pull request, applies labels, and updates the project board as before. The extraction and resolution steps were exercised locally against good, malformed and ambiguous inputs.

Checklist

• My pull request has a descriptive title (not a vague title likeUpdate index.md).
• My pull request targets the default branch of the repository (main) or a parent feature branch.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.
• I ran the DAG documentation generator (ov just catalog/generate-docs for catalog
  PRs) or the media properties generator (ov just catalog/generate-docs media-props
  for the catalog or ov just api/generate-docs for the API) where applicable.

Developer Certificate of Origin

Developer Certificate of Origin

Developer Certificate of Origin
Version 1.1

Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
1 Letterman Drive
Suite D4700
San Francisco, CA, 94129

Everyone is permitted to copy and distribute verbatim copies of this
license document, but changing it is not allowed.


Developer's Certificate of Origin 1.1

By making a contribution to this project, I certify that:

(a) The contribution was created in whole or in part by me and I
    have the right to submit it under the open source license
    indicated in the file; or

(b) The contribution is based upon previous work that, to the best
    of my knowledge, is covered under an appropriate open source
    license and I have the right under that license to submit that
    work with modifications, whether created in whole or in part
    by me, under the same open source license (unless I am
    permitted to submit under a different license), as indicated
    in the file; or

(c) The contribution was provided directly to me by some other
    person who certified (a), (b) or (c) and I have not modified
    it.

(d) I understand and agree that this project and the contribution
    are public and that a record of the contribution (including all
    personal information I submit with it, including my sign-off) is
    maintained indefinitely and may be redistributed consistent with
    this project or the open source license(s) involved.

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