fix: forward proxy settings to Docker builds

[JWThewes]

Summary

• detect non-empty standard proxy variables inherited through the central scripts/install.sh workflow and pass them as sensitive Terraform Docker build arguments
• forward build arguments to both AgentCore and Yjs Buildx builds while keeping the default non-proxy path unchanged and allowing explicit TF_VAR_docker_build_args overrides
• make the local AgentCore E2E harness proxy-aware and document managed-install, manual-deployment, state, and E2E behavior
• add focused coverage for proxy and non-proxy environments without exposing proxy values in Docker command arguments

Testing

• node --test scripts/test/docker-proxy-build-args.test.mjs scripts/test/agent-e2e-proxy.test.mjs (5 passed)
• live E2E_CLIS=claude,opencode ./scripts/agent-e2e-testing.sh (Claude passed, OpenCode passed; Kiro not selected because no key was available)
• Terraform validation using a backend-free copy
• terraform fmt -check -recursive terraform
• bash -n scripts/deploy-terraform.sh scripts/agent-e2e-testing.sh scripts/install.sh
• npm run lint
• npm run format:check
• npm run secretlint
• git diff --check

npm run test:release has one unrelated existing failure: the release test expects a TBD changelog date while the current changelog entry is dated. The pre-commit npm audit also reports existing dependency advisories; staged lint, format, and secret checks passed.

Fixes #347