docs: Replace placeholder README with comprehensive API documentation

[wheeyls]

Summary

The README has had TODO: Write usage instructions here since the gem was created. This replaces it with comprehensive documentation derived from reading the gem source and studying real usage patterns across 1000+ commands in the ue codebase.

What's documented

• Defining commands — include Command, option, execute
• Running commands — .run (monadic result) vs .run! (unwrap-or-raise)
• Declaring inputs — Required, optional with defaults, proc context
• Type system — Reference table of common Types:: usage (String, Integer, Bool, Instance, Interface, enum, Coercible, optional)
• Validations — ActiveModel validations before and during execute, errors.add
• Composing commands — compose with automatic error propagation via Command::Interrupt
• Input middleware — ActionController::Parameters handling, key symbolization
• API reference — Tables for class methods, instance methods, result objects
• Complete example — End-to-end with controller and job calling patterns
• RSpec helper — create_outcome for stubbing in tests

All examples are derived from real patterns in production (e.g., OrganizationAdmin::FindOrCreateCompetitors for compose, Votes::Exceeded for typed options with defaults).