fix: sync-preview restores version instead of ignoring files

[notgitika]

Summary

Improves on #1078 — instead of git checkout --ours to discard main's entire package.json/package-lock.json (which throws away new deps, scripts, etc.), this approach:

1. Saves preview's version before the merge
2. Merges main's content normally (getting all changes)
3. If there's a conflict on package.json/lock, accepts main's version then fixes the version field
4. After merge, surgically replaces only the version field back to preview's value
5. Commits that as a separate "restore preview version" commit for clarity

CHANGELOG.md and schemas merge normally from main — no special handling needed since they're autogenerated/diverge naturally.

Also keeps the structural improvements from #1078:

• Always creates a PR (respects branch protection, never pushes directly)
• Dedup check — skips if a sync PR is already open
• Cleaner step separation

Test plan

• Workflow YAML is valid (verified locally with yaml.safe_load)
• Push to main triggers sync-preview workflow
• Clean merge creates a PR against preview with version restored
• Conflicts in package.json/lock are auto-resolved (accept main, restore version)
• Other file conflicts produce a PR with conflict markers
• Existing open PR prevents duplicate creation

[agentcore-cli-automation]

Reviewed the workflow changes end-to-end. The approach is a clear improvement over #1078 — surgically restoring only the version field preserves new deps/scripts from main instead of discarding them, and splitting branch-creation/merge/PR-creation into distinct steps reads much better.

Verified the tricky bits:

• git add -A + git commit on conflicted files does stage and commit files with conflict markers as intended (confirmed locally), so the conflict-PR path produces a branch with markers for humans to resolve.
• git checkout --theirs + git add correctly removes package.json/package-lock.json from the unmerged set, so the subsequent --diff-filter=U check only sees remaining real conflicts.
• The version-restore node snippet reliably runs in both the clean-merge (main bumped version with no conflict) and conflict-resolved-by-theirs cases.
• Concurrency group + pre-merge Check for existing PR step correctly gates against duplicate sync PRs.

No blocking issues. LGTM.