During sync, the 3-way merge logic tries to delete issues that were
removed remotely. If an issue is already gone (tombstoned or never
existed locally), that shouldn't be an error - the goal is just to
ensure the issue is deleted.
Changes:
- Add isIssueNotFoundError helper to detect missing issue errors
- Skip "issue not found" errors during merge deletion (count as success)
- Update stats output to show already-gone count when relevant
e3d8119f8e