Create PR: Open the Pull Request, Hand Off for Review

This is the ship step of the PIV loop: the implementation is committed on a feature branch; now open the PR so it can be reviewed (by the piv-review-pr agentic gate, then a human).

Phase 0 — Detect the base branch

Don't hardcode main. Resolve it:

1. If $ARGUMENTS contains --base <branch>, use that.
2. Else: git symbolic-ref refs/remotes/origin/HEAD 2>/dev/null | sed 's@^refs/remotes/origin/@@'
3. Fallback: git remote show origin 2>/dev/null | grep 'HEAD branch' | awk '{print $NF}'
4. Last resort: main. Store as {base}.

Phase 1 — Validate git state