github_merge_pr: add optional subject/body params (gh pr merge --subject/--body) #44

Problem

github_merge_pr (write_tools.py, ~L287–330) builds gh pr merge <n> --repo <r> --<method> [--delete-branch] and offers no way to set the merge commit's subject or body. If the PR has a single commit, GitHub's squash uses that commit's subject, not the PR title. So correcting the PR title before merging doesn't stop a bad subject from landing on main, and there's no way to fix it inside the tool.

Evidence: design-system-plugin#48

• The PR title was corrected to chore: release v0.16.0 before merge.
• The single-commit squash still landed on main as feat: chore: release v0.16.0 (#48) (main 4a779cb).
• The release workflow keys on a chore: release v… subject, so release run 39870343120 was skipped.
• We recovered by hand with workflow_dispatch (run 39870464197).

The bad feat: prefix comes from projectBoard-plugin#566 / #567, which add a conventional prefix to a title that already has one. Fixing that removes this particular cause. This issue is about the tool: it is still the only in-tool way to correct a bad squash subject before it reaches main, whatever the cause.

Proposed direction

• Add optional subject: str = "" and body: str = "" params to github_merge_pr.
• When non-empty, append --subject <subject> / --body <body> to the gh pr merge args. These are gh's existing flags; --body-file isn't needed.
• Allow them for squash and merge. For rebase, refuse with a clear error, since gh/GitHub ignore a subject on rebase merges and the call would otherwise appear to succeed.
• Include them in the dry_run preview, e.g. … via squash with subject "chore: release v0.16.0 (#48)", so the operator can confirm the exact subject before confirm=true.
• Optionally add them to the pr.merged event payload.
• Update the docstring to say that a single-commit squash uses the commit subject, not the PR title, unless subject is set.

Acceptance

• github_merge_pr(number=N, method="squash", subject="chore: release vX.Y.Z (#N)", confirm=True) runs gh pr merge N --repo R --squash --subject "chore: release vX.Y.Z (#N)".
• body maps to --body in the same way. Empty values add no flags, so current behaviour is unchanged.
• method="rebase" with a non-empty subject or body returns an Error: and does not run gh.
• The dry-run output shows the subject/body when set.
• Unit tests cover the arg construction for all of the above.

Refs: projectBoard-plugin#566, projectBoard-plugin#567, design-system-plugin#48