No token, no auto-PR — not nikhilsoman

The Broader Goal at the End of the Previous PR

#1453 shipped synlynk gh --role. Parent auto-PR already injected a role token when one existed (#1452) but still fell through to host gh when it did not.

Strategic Shifts in This PR

None. Spec #1436: fail-closed skip of host gh for parent auto-PR unless SYNLYNK_GH_WRITE_ALLOW_HOST_AUTH=1.

What This PR Shipped

_maybe_open_worktree_pr skips gh pr list / gh pr create when the role App token is missing and the host-auth override is unset.

Brainstorm Visuals Used

None.

What This Achieved on the Path to Autonomy

A job without a cached App token no longer opens a PR as nikhilsoman.

Strategic Note: The Goal at the End of This PR

#1436 still open for optional qa actions: write, a live qa --approve of a bot-authored PR, and making synlynk gh the default instead of raw host gh.