Add open in browser to create-pr skill

[ydesgagn]

Summary

Make the create-pr skill open the newly-created PR in the default browser so callers (interactive use, repos.sh) get a single consistent open path.

Key changes:

• Add Bash(open:*) to allowed-tools so Claude can invoke open.
• Step 5 wraps gh pr create in PR_URL=$(...) and adds open "$PR_URL".
• Pre-existing uncommitted skill rewrite (Steps 3–6 end-to-end flow) is included so the skill can actually create the PR it opens.