ci: re-enable optional ABI snapshot check

[dreamgene]

Summary

• add a dedicated GitHub Actions workflow to run contract/scripts/snapshot-abi.sh --check on contract PRs
• keep the ABI snapshot job non-blocking with an optional workflow job
• update the ABI snapshot README to document manual check and regeneration flows

Testing

• bash -n contract/scripts/snapshot-abi.sh
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/abi-snapshot.yml")'

Closes #600

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