open-pr - compose, verify, and open a PR

Phase 0 - Sanity checks

Before doing anything:

# Where are we, where's the base, what's the state?
git status --short
git rev-parse --abbrev-ref HEAD
git rev-list --count HEAD ^origin/<main>  # commits ahead of main
git rev-list --count origin/<main> ^HEAD  # commits behind main
gh pr view --json number,state 2>/dev/null  # does a PR already exist?

Hard gates: