feat: Enable sprout to work without .env.example files

[laysakura]

Summary

• Modified sprout create to work in repositories without .env.example files
• Changed error behavior to show warning instead of exiting when no .env.example files found
• Updated documentation to reflect that .env.example files are optional
• Added comprehensive test coverage for new functionality

Test plan

• Unit tests pass for repositories without .env.example files
• Integration tests verify end-to-end functionality
• Existing functionality with .env.example files remains unchanged
• Documentation updated to reflect new behavior
• All existing tests continue to pass

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