Generator: use the @AckModel class doc comment as the object schema description #175

Problem

@AckModel() turns each field's doc comment into that field's schema description, but it ignores the class doc comment. The generated object schema has no description:

/// A task the person can complete.   // not carried today
@AckModel()
final class Task with _$TaskAck {
  const Task({required this.id, required this.title});

  final String id;

  /// What to do.                      // carried today
  final String title;
}

TaskSchema.toJsonSchema()['description']; // null
TaskSchema.schema.description;            // null

@AckModel() has no other way to set an object description, and @AckField has no description: parameter. We pass these schemas to a language model to describe app state, so today we have to repeat each class description by hand next to the model.

Proposal

When an @AckModel() class has a doc comment, use it as the description of the generated object schema:

• Normalize it the same way as field docs: /// and /** */, with the comment markers removed and the lines joined by spaces.
• Apply it to the generated object schema and to the model codec, so XSchema.toJsonSchema(), XSchema.schema.description, XSchema.wireSchema.description, and toSchemaModel() all carry it.
• Unions: the sealed base's doc comment describes the discriminated schema, and each branch's doc comment describes that branch's object in anyOf. Implicit, unannotated branches work the same way.
• With no doc comment, no description is emitted. The output for undocumented classes doesn't change.
• When a model is used as a field of another model and the field has its own doc comment, the field doc wins at that property, the same way .describe() layers today. Without a field doc, the nested model's class description appears there.

@AckInfer() needs no change, because its schema is authored directly and .describe() already covers it.

Compatibility

This is additive: no annotation or API changes. After regeneration, classes that already have doc comments gain a description in their generated schema, just as documented fields already do.