Implement configurable PasswordIdentity schema factory

[dpickett]

Summary

Implements a flexible schema factory for password-based authentication that supports configurable identifier types with compile-time type safety.

Changes

• ✨ Added createPasswordIdentitySchema factory function
• 🔧 Support for email, username, and phone identifiers
• 📝 Compile-time type safety with as const arrays
• 🧪 Comprehensive test coverage for all combinations
• 📚 Updated user story documentation

Usage

// Email only
const schema = createPasswordIdentitySchema(['email'] as const);

// Multiple identifiers with custom validation
const schema = createPasswordIdentitySchema(['email', 'phone'] as const, {
  phone: z.string().regex(/^\+1[0-9]{10}$/)
});

Test Plan

• All identifier combinations tested
• Custom schema validation verified
• Type safety confirmed at compile time
• Documentation updated with examples

Closes #2

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