Add Cursor rule: enforce GitHub issue creation and closeout workflow

[r2DoesInc]

Summary

Add a Cursor rule (.cursor/rules/mdc-gh-issue-workflow.mdc) that enforces the following workflow for every unit of planned work:

1. Issue first, code second — a GitHub issue must be created via gh issue create before any implementation begins.
2. Commit closes the issue — the final commit message must include Closes #<number> (or Fixes / Resolves) so the issue is automatically closed when the commit lands on the default branch.
3. One issue per plan — unrelated changes should not be batched under a single issue.

Acceptance criteria

• .cursor/rules/mdc-gh-issue-workflow.mdc exists with alwaysApply: true
• Rule content documents the workflow with examples for gh issue create and commit message format
• Rule is concise (<50 lines of content)