🧪 [testing] add onboard flags unit tests

[Hardonian]

🎯 What: Missing tests for buildOnboardFlags in src/lib/commands/onboard/common.ts
📊 Coverage: Validates flag definitions, exclusivity, options vs booleans, and tests toLegacyOnboardArgs to ensure flags map correctly to cli arguments.
✨ Result: 100% test coverage for the onboard flags and backwards-compatibility argument generation logic.

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PR created automatically by Jules for task 4256092367230418622 started by @Hardonian

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