feat: auto-register Complete<FieldName> flag completions

[leodido]

Summary

• auto-discover and auto-register optional Complete<FieldName> hooks during Define() for generated flags
• validate completion hook signatures and return a dedicated validation error for invalid signatures
• add coverage for completion registration and validation behavior
• document the new hook pattern and update the full example to use CompleteTargetEnv

Validation

• go test ./...
• cd examples/full && go test ./...