Support App Runner web deploys from ARM runtimes

[ZILECAO]

Summary

• build the Next.js application on the Docker host architecture instead of running Turbopack through QEMU
• install the target architecture's traced Sharp runtime dependencies before assembling the App Runner image
• include Docker Buildx in newly built AgentFormation runtime AMIs and verify it during image creation

Why

AgentFormation defaults to ARM runtime instances, while App Runner consumes the published linux/amd64 web image. A full cross-build ran Next.js/Turbopack through QEMU and failed with SIGSEGV, preventing an AgentFormation runtime from deploying its own frontend. The final image remains linux/amd64; only the build stage now runs natively.

Validation

• ./scripts/check.sh
• 48 web tests, format, lint, typecheck, production build, ShellCheck, and CloudFormation lint all pass
• ARM-to-linux/amd64 image build and runtime health validation will be recorded before merge

No authentication, user isolation, runtime stack, or model configuration behavior changes.

[ZILECAO]

@claude Please review this focused ARM deploy fix. The full repository check passes. I also built the exact PR commit on an m7g.xlarge ARM AgentFormation runtime with Buildx targeting linux/amd64: Next.js compiled natively, the target-stage require("sharp") check passed under BuildKit emulation, and Docker produced sha256:67cd98445b4ca54d2a586a1a2211cceaa49830795a81cea6559ce7e97961324b with architecture=amd64 and cmd=["node","start.mjs"]. Please call out any blocker or nit; an explicit LGTM is helpful if clean.