Herald: Implement RabbitMQ Message Bus

[jorisvilardell]

Priority: High
Dependencies: None

Description:
Implement the MessageBusRepository trait using RabbitMQ via AMQP protocol to publish action events.

Acceptance Criteria:

• Create RabbitMqMessageBus in infrastructure/messagebus/ module
• Implement publish() to send ActionEvent to RabbitMQ
• Use routing key from ActionEvent (format: {resource}.{kind})
• Handle connection failures and implement retry logic
• Map AMQP errors to HeraldError::MessageBus
• Add configuration for RabbitMQ connection (host, port, credentials, exchange)
• Integration tests with testcontainers

Technical Notes:

• Use lapin or amqprs crate for RabbitMQ
• Serialize ActionEvent to JSON for message body
• Use topic exchange for routing flexibility
• Ensure idempotent publications (include action_id in message)