feat: implement causes() function for cause chain traversal

[martyy-code]

Summary

Add causes(error) function that returns the causes array from an error:

consterr=ValidationError({field: 'email'}).from(newNetworkError('Connection failed'));// causes(err) returns [NetworkError]

Features

• Returns array from most recent to root cause
• Returns empty array for error with no cause
• Works through multiple chaining levels
• Works with native errors in chain
• err.causes property already works (from Task 05)

Implementation

The function is straightforward - it returns the causes array stored on the error instance by .from(). This avoids code duplication and maintains consistency.

Test plan

• 78 tests passing
• TypeScript strict mode passes
• ESLint passes

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