Why

Running pnpm cella sync again before merging the previous sync PR silently stacks changes: the last-sync point lives in cella.manifest.json committed on the sync branch, so a new cycle cut from a trunk that still lacks the merged PR re-includes that PR's entire delta on top of the new upstream commits — the "why are there suddenly so many changes" surprise.

Two tweaks address this.

1. Pre-flight guard for an unmerged sync PR

Before cutting a fresh sync branch, cella sync now checks for an open sync PR (head branch under cella/sync/) via gh and prompts:

⚠ an earlier sync PR is still open and unmerged:
  #42  chore: sync upstream cella v0.3.0 (abc1234)
         https://github.com/org/app/pull/42
  starting a new sync now produces a PR that re-includes those changes.
? how do you want to proceed?
  ❯ merge #42 first, then sync   (recommended)
    continue anyway   (new PR will re-include the unmerged changes)
    cancel

• merge first → gh pr merge --squash --delete-branch the open PR, then fast-forward the local trunk so the new cycle cuts clean. A PR GitHub refuses to merge (conflicts / failing required checks — the "breaking changes" to fix first) stops the run with guidance.
• continue → today's behaviour.
• cancel → bail.

Multiple stacked sync PRs: the newest is a content superset, so it is merged and the older ones are closed.

Degrades to a silent no-op when it can't help: no gh, no open sync PR, or a non-interactive session (no TTY).

2. --direct-merge flag

Once the sync PR is open, enables GitHub auto-merge (squash) so it merges itself as soon as required checks pass — no forgotten open PR to trip over next time. Opt-in per run (deliberately not a config default — a sync PR is usually worth a human look), and needs auto-merge enabled on the repo. Falls back to printing the manual command if the repo does not allow it.

release-please is unaffected either way: it watches the trunk for the single squashed chore: sync upstream cella … commit, which lands identically whether the PR is merged by hand, by the guard, or by auto-merge (the branch is already flattened to one commit before pushing).

Implementation

• New src/utils/gh.ts consolidates the scattered gh calls behind small wrappers (ghAvailable, listOpenSyncPrs, mergePrSquash, closePr) with pure, unit-tested cores (filterSyncPrs, buildMergeArgs).
• src/services/sync.ts: guardAgainstOpenSyncPr / mergeOpenSyncPrs in the fresh-cycle path; enableAutoMerge in shipSyncBranch.
• --direct-merge wired through cli.ts + RuntimeConfig.

Tests

• tests/gh.test.ts — filterSyncPrs (prefix boundaries) and buildMergeArgs (flag combinations).
• tests/cli.test.ts — sync --direct-merge parsing.
• Full suite green (14 files, 144 tests); pnpm ts + pnpm biome check clean; verified sync --help lists the flag after pnpm build.

The gh-network paths degrade to a no-op without gh, which is exactly the e2e condition, so the existing e2e suite is untouched.

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