Populate whenToUse for built-in modes

[mrubens]

This makes more sense to me than just falling back on the first 100 characters of the role definition... thoughts?

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Important

Adds whenToUse descriptions for built-in modes in modes.ts, enhancing clarity on mode usage.

• Behavior:
  • Adds whenToUse descriptions for built-in modes in modes.ts.
  • Provides specific guidance for code, architect, ask, debug, and orchestrator modes on when to use them.
• Misc:
  • Fixes a typo in system.test.ts.snap.

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