docs: record empty-branch PR workflow note (#3065)

## Summary - document that `git rev-list --left-right --count upstream/main...HEAD` returning `0 0` means the branch cannot produce a PR - note that agents should report the branch as pushed but PR-blocked instead of creating an empty commit just to satisfy `gh pr create` - update the git workflow rules with this edge case ## Test plan - npm run fmt - npm run lint:fix - npm run ts - npm test #skip-bugbot 🤖 Generated with [Claude Code](https://claude.com/claude-code)  --- <a href="https://app.devin.ai/review/dyad-sh/dyad/pull/3065" target="_blank"> <picture> <source media="(prefers-color-scheme: dark)" srcset="https://static.devin.ai/assets/gh-open-in-devin-review-dark.svg?v=1"> <img src="https://static.devin.ai/assets/gh-open-in-devin-review-light.svg?v=1" alt="Open with Devin"> </picture> </a>  Co-authored-by: Will Chen <7344640+wwwillchen@users.noreply.github.com>

docs: record empty-branch PR workflow note (#3065)
0ab62856 · wwwillchen-bot · GitHub · 871e3b41 · 0ab62856
--- a/rules/git-workflow.md
+++ b/rules/git-workflow.md
@@ -24,6 +24,16 @@ gh pr create --head <owner>:<branch> ...

 This can happen when remotes are configured in a non-fork layout and `gh` fails to infer the branch mapping.

+## Empty branches cannot produce PRs
+
+Before creating a PR for a freshly pushed branch, check whether it is actually ahead of the base branch:
+
+```bash
+git rev-list --left-right --count upstream/main...HEAD
+```
+
+If this returns `0	0`, the branch has no commits ahead of `upstream/main`. GitHub cannot open a PR for an empty branch, so do not fabricate an empty commit just to satisfy `gh pr create`; report the branch as pushed but PR-blocked instead.
+
 ## `gh pr create` body quoting

 When passing a PR body inline via `gh pr create --body "..."`, unescaped backticks are evaluated by `zsh` before `gh` runs. Avoid backticks in inline bodies, or use a body file / heredoc so literal code identifiers do not turn into `command not found` errors.