#48 - Add optional AWS availability zone selection

[spacepirate0001]

What

Adds an optional availability zone selection for AWS deployments so users can route around per-zone GPU capacity shortages.

Closes #48.

Why

The AWS subnet, and therefore the EC2 instance, was always placed in the first sorted availability zone that offers the requested instance type (e.g. us-west-2a):

availability_zone = try(sort(data.aws_ec2_instance_type_offerings.zones.locations)[0], "not-available")

AZ offerings indicate that a zone supports an instance type, not that it currently has capacity. As a result, deployments deterministically landed in the same zone and failed with InsufficientInstanceCapacity whenever that zone was exhausted, even when other zones in the region had capacity. Re-running did not help because the selection is deterministic.

How

• New optional availability_zone Terraform variable on both the root aws module and the isaac-workstation module (default "").
• The subnet now uses the requested AZ when provided, otherwise falls back to the existing auto-select (first offered zone), so default behavior is unchanged.
• New --availability-zone / --az option on deploy-aws. It accepts a full zone name (us-west-2b) or a bare suffix letter (b, combined with the chosen region), validates that the zone belongs to the selected region, and writes the value into the tfvars passed to Terraform.

Backward compatibility

Leaving the prompt empty (the default) reproduces the previous behavior exactly. Existing .tfvars files without the variable are fine because the variable defaults to "".

Testing

• python3 -c "import ast; ast.parse(open('deploy-aws').read())" passes.
• Verified the existing tests do not assert on CLI param counts or tfvars keys, so they are unaffected.
• Note: terraform fmt/validate should be run in the project container.

Files changed

• deploy-aws
• src/terraform/aws/main.tf
• src/terraform/aws/variables.tf
• src/terraform/aws/isaac-workstation/main.tf
• src/terraform/aws/isaac-workstation/variables.tf