feat(gen2-migration): implement clean working directory validation for gen2 migration

[sai-ray]

Description of changes

This PR implements working directory validation to ensure the git repository is in a clean state before proceeding with Gen 1 to Gen 2 migrations. The implementation adds a validateWorkingDirectory() method that uses execa to execute git commands, checking for uncommitted changes via git status --porcelain and unpushed commits via git log @{u}.. --oneline. When validation fails, it throws MigrationError with clear messaging to guide users on resolution steps. The implementation gracefully handles cases where no upstream branch is configured, allowing migrations to proceed in such scenarios.

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Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• Pull request labels are added

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.