fix: sync-upstream should merge, not rebase, so future sync PRs stay mergeable

[SmailG]

Summary

Follow-up to #1 and #2. Once those landed, the workflow produced a real sync PR (#3) — but it opened with a merge conflict against main. I resolved #3 by hand and merged it (main is now caught up to upstream 3fe2823), and this PR fixes the workflow so future sync PRs don't hit the same wall.

Root cause

Create sync branch and rebase took whatever commits were unique to main (relative to upstream/main) and rebased them onto the new upstream tip, producing a branch with entirely rewritten commit SHAs. That's fine in isolation, but the moment main gains a commit that didn't come from a previous sync merge (here: the #1 and #2 workflow fixes), the rebased branch and main no longer share recognizable ancestry — git sees independent "adds" of the same fork-only files (.claude-plugin/.upstream-base, in this case) with no common version to 3-way-merge from. That's an add/add conflict, and it blocks every GitHub merge strategy (merge, squash, and rebase), not just the one the workflow was using.

This means the rebase approach was only ever going to produce a cleanly-mergeable PR the very first time. Any future manual commit to main (a workflow tweak, a hotfix, anything) would reintroduce the same conflict on the next sync.

Fix

Switch branch construction from git rebase upstream/main to git merge upstream/main:

• The sync branch is now a genuine descendant of main's actual tip (upstream/main merged in as a second parent), so it can't lose ancestry with main the way a rebase does — the next sync only ever needs to merge the delta since the last one.
• Same conflict policy as before ("upstream wins", README rebuilt from the fork's template), just with --ours/--theirs flipped to match merge semantics (they're inverted relative to rebase).
• .claude-plugin/.upstream-base no longer conflicts at all going forward — upstream doesn't have that path, so it never participates in the 3-way merge; it's written as a deliberate step after the merge instead.
• Final gh pr merge --auto switches from --rebase to --merge, since GitHub's rebase-merge rejects any branch containing a merge commit — which this branch now always has by design.

Validation

I hand-resolved #3 using this exact merge direction (merge main into the sync branch, keep the newer .upstream-base value, everything else merged with zero conflicts) before writing this fix, so the conflict-resolution mechanics are proven against real data. Upstream hasn't produced new commits since 3fe2823 yet, so I haven't been able to trigger a fully live end-to-end run of the new code path — happy to trigger workflow_dispatch once this merges and there's new upstream content to sync, to confirm the automated run produces a cleanly-mergeable PR with no manual intervention.

Test plan

• YAML parses successfully.
• Conflict-resolution direction/policy validated by hand against chore: sync with cursor/plugins — pstack: parity sweep with the private skill tree (#156) #3's actual conflict.
• Next real upstream change produces a sync PR that merges without intervention.

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