fix for rebase in sync cmd

[skarim]

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

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[Copilot]

Pull request overview

This PR fixes gh stack sync so it performs a cascade rebase when stack branches are stale relative to trunk, even if trunk itself was already updated locally before running sync. It also reduces duplication by extracting shared trunk fast-forward / original-ref resolution / cascade rebase logic into common helpers and aligns rebase behavior with sync by skipping queued branches too.

Changes:

• Add stackNeedsRebase() and use it in sync to trigger cascade rebases even when trunk wasn’t fast-forwarded in the current run.
• Extract shared helpers (fastForwardTrunk, resolveOriginalRefs, cascadeRebase) into cmd/utils.go and refactor sync/rebase to use them.
• Update tests to cover the “trunk up-to-date but stack stale” scenario and basic stackNeedsRebase cases.

Show a summary per file

File	Description
cmd/utils.go	Adds shared helpers for original ref resolution, trunk FF, cascade rebase, and stale-stack detection.
cmd/utils_test.go	Adds unit tests for stackNeedsRebase().
cmd/sync.go	Uses shared helpers and changes rebase gating to include stale-stack detection.
cmd/sync_test.go	Adds regression test ensuring stale stack triggers rebase even when trunk is already up-to-date.
cmd/rebase.go	Refactors to use shared helpers; updates skip logic to include queued branches; reuses cascade helper in --continue.

Copilot's findings

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• Files reviewed: 5/5 changed files
• Comments generated: 3