gh stack merge retries against an unmerged base PR, then silently rebases and dismisses approvals on the dependent PR

**Phase 1 — 3 failed attempts before anything merged (15:26–15:31 UTC):**
Triggered "Squash and merge stack" from the PR #111 web panel three times. Each attempt logged a real `auto_merge_disabled` event on **PR #111 only** (15:26:58, 15:27:55, 15:30:34) — none on PR #104, the actual bottom-of-stack PR that must merge first. Per the README, `gh stack merge` is meant to be "all-or-nothing," but it appears to arm merge/auto-merge on the dependent PR before confirming the base PR has landed, hits an unresolved `mergeable` state, and aborts instead of waiting or retrying automatically — surfacing as a confusing "not mergeable" error with no indication that nothing had actually merged.

**Phase 2 — merge manually, but silently strips approvals (15:35:23–15:35:29 UTC):**
PR #104 squash-merged successfully. Immediately after:
- PR #111 was rebased — all 3 commits got new SHAs with identical timestamps (a full rewrite, not just a base pointer change), then force-pushed.
- **All 3 existing approving reviews were dismissed** (confirmed via `review_dismissed` events), reverting `reviewDecision` to `REVIEW_REQUIRED`.
- Every CI check reset to `pending`.

Our org's ruleset has `dismiss_stale_reviews_on_push: false` — this dismissal happened regardless, because it was a history-rewriting force-push rather than an append-only push (which GitHub dismisses reviews for unconditionally). That's expected GitHub behavior for *that specific push*, but the push itself was an undisclosed side effect of the merge action — the PR content is byte-identical to what was approved, yet 3 reviewers now have to re-approve and CI has to fully rerun, with zero warning in the UI before this happened.

**Ask:** `gh stack merge` should either (a) confirm the base PR is actually merged before touching the dependent PR, and (b) warn — or offer a non-rebasing retarget path — before force-pushing an already-approved PR as a side effect of merging the layer beneath it.

Start at the gh stack merge command and the README behavior described in the issue; reproduce the flow with a stacked pair like PRs #104 and #111, then inspect the merge, rebase, review, and check events. Done means the command handles the unmerged base safely and makes any force-push side effect explicit before changing an approved dependent PR.

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