Added aws_create_instance.sh script and tests

[salvatoredipietro]

Summary

• Adds scripts/aws_create_instance.sh, an env-var-driven wrapper around aws ec2 run-instances for spinning up a throw-away SUT/LDG instance for repro/benchmark work. It is explicitly out of scope of the Repro Framework (infrastructure management remains the caller's responsibility) and is documented as such.
• Adds a full test suite under tests/: 74 bats unit tests (no real AWS calls, using a fake_aws.bash stub) and a real-AWS integration test script with automatic cleanup.
• Updates README.md to document the new scripts/ and tests/ directories and clarify the infrastructure-management scope exception.

What aws_create_instance.sh does

• Accepts all configuration via environment variables (INSTANCE_TYPE, KEYPAIR, EXPNAME, AMI, EBS_DISKS_*, SUBNET_ID, SG_ID, PLACEMENT_GROUP_NAME, TENANCY, …). See --help for the full list.
• Auto-discovers AMI, subnet, and security group from the target account when not provided.
• Supports AMI aliases (AL2023_K6.12, AL2023, UBUNTU2404, etc.) in addition to raw ami-* IDs, and infers the correct SSH user and root device from the AMI metadata.
• Optionally attaches up to 25 EBS data volumes (gp3/gp2/io1/io2/st1/sc1) and configures the root volume size, type, and IOPS.
• Supports placement groups (creates them on first use), dedicated tenancy, public IP association, and instance tagging.
• Clones the repro-collection repo into the instance via cloud-init on boot (CLONE_REPO=true by default).
• Writes a resource JSON (repro-results/${EXPNAME}.resources.json) for downstream scripts to pick up instance IDs, IPs, subnet, and AZ.
• DRYRUN=true validates and prints the run-instances call without launching anything.

Tests

Layer	File	Coverage
Unit (bats)	tests/unit/scripts/aws_create_instance.bats	74 tests — input validation, arch inference, AMI resolution, subnet/SG discovery, block-device mappings, tenancy/tags, user-data, DRYRUN, placement groups, full launch summary
Unit helper	tests/unit/helpers/fake_aws.bash	aws CLI stub, fully isolated via env -i, captures exact run-instances argv
Integration	tests/integration/aws_create_instance_integration.sh	Real-AWS end-to-end: arm64 minimal, x86 custom root, arm64 data disks, placement group; auto-terminates all created resources on exit/interrupt

Test plan

• bash -n scripts/aws_create_instance.sh passes
• Integration test dry-run: DRYRUN=true INSTANCE_TYPE=t4g.micro KEYPAIR=<key> EXPNAME=test scripts/aws_create_instance.sh
• Full integration: AWS_PROFILE=default AWS_REGION=us-west-2 tests/integration/aws_create_instance_integration.sh --yes

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