ci: Annotate PR descriptions with branch and user info

[AhooPao]

Summary

Two-part change so that every squash-merge commit's Extended description includes source/target branch and user info:

1. A new workflow (.github/workflows/pr-branch-info.yml) appends a Branch info section to every PR description when the PR is opened:

   • Source user / source branch (head repository owner and head ref)
   • Target user / target branch (base repository owner and base ref)

2. Required repository setting change (needs admin, see below): set squash_merge_commit_message to PR_BODY so the merge dialog's Extended description is pre-filled with the PR description instead of the commit list. This is what carries the Branch info section into the squash commit message at merge time.

Required repository setting (admin)

With the current settings (squash_merge_commit_message=COMMIT_MESSAGES), the Extended description at merge time is the commit list, and PR description content is dropped — so the branch info would never reach the merge commit.

Please change it via Settings → General → Pull Requests → Allow squash merging → Default commit message → "Pull request title and description", or run:

gh repo edit BossZou/jaclks --squash-merge-commit-message pr-title-description

Motivation

For PRs (especially from forks), it is useful to see in the merge commit who owns the source branch and which branches the merge involved.

Related issue

Closes #64

Test plan

• Workflow YAML and shell syntax validated locally.
• Script logic simulated locally with a mock gh CLI:
  • appends the section to empty and existing bodies
  • skips when the section is already present (idempotent)
  • hostile branch/user names are written literally (no command injection)
• Note: the workflow first runs on PRs opened after this is merged (new workflow files do not run on the PR that adds them).

Checklist

• CHANGELOG.md updated (CI-only change; entry added)

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.14%. Comparing base (4c38672) to head (d4fe321).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #65   +/-   ##
=======================================
  Coverage   93.14%   93.14%           
=======================================
  Files          25       25           
  Lines         831      831           
  Branches      271      271           
=======================================
  Hits          774      774           
  Misses         32       32           
  Partials       25       25           

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