fix: sync performs cascade rebase even when trunk is already up-to-date

Previously, `gh stack sync` gated the cascade rebase on whether trunk
or stack branches were fast-forwarded during the current run. This meant
that if the user had already updated trunk locally (e.g., `git pull`),
sync would skip the rebase entirely even though stack branches hadn't
been rebased onto the current trunk.

This change:
- Adds `stackNeedsRebase()` to detect stale branches regardless of
  whether trunk was updated in this run
- Extracts shared helpers (`fastForwardTrunk`, `cascadeRebase`,
  `resolveOriginalRefs`) from duplicated code in sync.go and rebase.go
  into utils.go, reducing ~450 lines of duplication
- Fixes rebase.go to skip queued branches (was only skipping merged),
  consistent with sync's behavior via `IsSkipped()`
- Refactors rebase --continue to reuse the shared cascade helper

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: skarim