feat: add live sandbox execution path

[Ker102]

Summary

• make LocalStack sandbox startup detached, named, localhost-bound, and removable with sandbox down
• add sandbox up --env-file so LocalStack auth tokens can stay outside Git and terminal output
• add live Terraform apply/re-apply path for Terraform-backed LocalStack scenarios
• add MI300X model-serving scripts and runbook for Qwen3.5 on SGLang and Gemma 4 on vLLM ROCm

Verification

• bash -n scripts/droplet/serve-qwen35-sglang-rocm.sh scripts/droplet/serve-gemma4-vllm-rocm.sh scripts/droplet/collect-endpoint-evidence.sh
• python -m ruff check src tests
• python -m mypy src
• python -m unittest discover -s tests -v
• python -m pip_audit . --skip-editable
• python -m nullstate sandbox up localstack-aws --dry-run --env-file .env.local
• python -m nullstate run --help

Runtime note

I attempted to start LocalStack AWS locally, but the current localstack/localstack image exits without LOCALSTACK_AUTH_TOKEN. This PR adds --env-file support so the real run can proceed with an untracked .env.local.

Refs #3

[coderabbitai]

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