refactor(terraform): decouple EBS volumes from EC2 instance resource — Layer 2

[bhumi46]

Part of #273

What

Replace inline ebs_block_device blocks inside aws_instance with separate aws_ebs_volume + aws_volume_attachment resources in their own Terraform root.

Why

EBS volumes defined as inline ebs_block_device blocks are owned by the aws_instance resource. Resizing or adding a volume requires modifying the instance resource itself, risking instance replacement.

Acceptance Criteria

• ebs component is a standalone Terraform root under terraform/implementations/aws/ebs/
• Uses aws_ebs_volume + aws_volume_attachment (not inline blocks)
• Volumes can be added, resized, or detached independently of EC2 instance lifecycle
• Reads EC2 instance IDs via data "aws_instance" by tag