ci: sync dev after main merges

[YunaBraska]

Summary

• Sync dev back to main after a merged PR into main, so squash merges do not leave dev historically haunted.
• Keep the automation inside the existing single workflow and single job.
• Add a manual sync_dev dry-run path for testing without pushing.
• Skip build, cache, artifact, and release upkeep during sync-only runs.

Checks

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/build.yml"); puts "yaml ok"'
• Shell syntax check for every workflow run: block after neutralizing GitHub expressions
• Local plan simulation for merged PR sync, manual sync dry-run, manual release, main push release, and release-commit skip
• Local sync dry-run against origin/main and origin/dev
• GitHub Actions PR/push runs on this branch
• GitHub Actions manual sync_dev=true, dry_run=true run on this branch