smart-rebase — rebase onto the base branch and force-push

The whole workflow is in scripts/rebase-sync.sh, split into subcommands so a conflict can stop the run, get fixed, and resume. Drive the script — don't hand-roll git fetch/rebase/push calls, because the script also carries the safety checks (refuse on the base branch, lease-protected push, autostash).

$SKILL_DIR below is this skill's directory.

Two paths: stacked PRs vs. a plain branch

preflight reports a STACK: line, and it decides everything that follows:

• STACK: stack — the branch belongs to a GitHub stacked PR chain and gh-stack is installed. Take the stack path (stack-sync).
• STACK: no-stack or no-extension — take the plain path (rebase → push).