docs(branching): fix the promotion template ancestry precondition

[ss-o]

Summary

Fixes the promotion precondition in .github/PULL_REQUEST_TEMPLATE/promotion.md,
which required a check that cannot pass after any successful promotion.

Closes #461

Problem

Line 19 of the promotion readiness record required:

- [ ] `git merge-base --is-ancestor <prior-main> <candidate-next>` succeeds.

Promotion creates a merge commit on main that never reaches next, so main
stops being an ancestor of next as soon as a promotion lands. The template
then instructed the maintainer to stop and reconcile, on every promotion,
including healthy ones.

Three no-content reconciliation merges have already landed here as a result:
9a4fa51, b0ebcc3, and 73df7bf. git diff --name-only <merge>^1 <merge>
returns zero files for each.

Change

Replaces the checklist item with an empty three-dot content diff and corrects
the accompanying guidance to identify a genuinely omitted commit with
git log --oneline <candidate-next>..<prior-main>.

Verification

Scenario	old check	new check	Correct
Just promoted, healthy	fail	pass	pass
Work continued on next, healthy	fail	pass	pass
Unmerged hotfix on main	fail	fail	fail
Hotfix synchronized forward	pass	pass	pass

The third row confirms a genuine stable-only commit is still caught, so this
removes a false negative without weakening the gate.

Against the live pending candidate:

git merge-base --is-ancestor origin/main origin/next   ->  fails
git diff --name-only origin/next...origin/main         ->  empty

trunk check reports no issues on the changed file.

Dependency

The canonical rule is corrected in z-shell/.github#574, which closes
z-shell/.github#573. This repository-local template must be fixed too, or the
impossible check is reimposed on the promotion pull request even after the
runbook is correct.

Both should land before the pending next to main promotion.