commit-push-pr: include GitHub issue-closing keywords in PR body

[lightcap]

Problem

The commit-push-pr command (commit-commands plugin) creates PRs with gh pr create but doesn't prompt for or include GitHub issue-closing keywords (Closes #123, Fixes #123, Resolves #123) in the generated PR body.

This means issues that are fully resolved by the PR don't get auto-closed on merge, requiring manual cleanup.

Example

A PR implementing issue #29 was created without any closing keyword reference. The issue had to be manually closed after the PR was merged.

Suggested change

In plugins/commit-commands/commands/commit-push-pr.md, the PR creation step could:

1. Check whether any related issues exist (e.g., by scanning commit messages or branch names for issue numbers)
2. Prompt the agent to include a closing keyword line in the PR body, e.g.:

Closes #<issue-number>

Context

GitHub supports these closing keywords in PR bodies (case-insensitive): close, closes, closed, fix, fixes, fixed, resolve, resolves, resolved. When a PR with these keywords targeting an issue is merged into the default branch, the issue is automatically closed.