fix(cmd): only push stack branches when submit will touch PRs (#89)

fix(cmd): defer submit push until PR intent is known

Submit used to force-push every stack branch before the PR phase, so
branches were published even when the user skipped opening a PR.

Plan each branch first (GitHub adopt check, prompts, --update-only),
then push only branches that will get PR work or must exist on the
remote as a base for a descendant PR. PR execution reuses the plan.

Add a unit test for ancestor push propagation on skip.

Co-authored-by: Cursor Agent <cursoragent@cursor.com>

Author: boneskull