Keep production Modal workers warm

[anth-volk]

Fixes #1525

Summary

• Keep a small warm worker pool for Modal main with min_containers=3, buffer_containers=2, and scaledown_window=600.
• Leave non-main Modal environments without always-on workers.
• Require MODAL_ENVIRONMENT during Modal release deploys and pass it through to worker app module import.
• Clean up retired Modal worker apps by default in the weekly release shape and workflow dispatch default.
• Update AI-facing Modal release guidance and the weekly update PR helper to use cleanup_target: retired.
• Add focused tests for worker options, missing deploy environment validation, and weekly default cleanup behavior.

Testing

• make format-check
• uv run --extra dev pytest -vv --timeout=150 -rP tests/unit/modal_release/test_worker_app.py .github/scripts/test_modal_deploy_release.py
• uv run --extra dev pytest -vv --timeout=150 -rP .github/scripts/test_resolve_modal_release_config.py tests/unit/modal_release/test_release_config.py tests/unit/modal_release/test_manifest.py .github/scripts/test_check_modal_release_body.py
• bash -n .github/scripts/create_pr.sh
• ruff check .github/scripts/test_resolve_modal_release_config.py policyengine_household_api/modal_release/release_config.py

Notes: make format-check printed a local Docker daemon warning while Make expanded docker helper variables, but ruff format --check . completed successfully. Repo-wide ruff check . currently fails on pre-existing lint issues outside this change set.