feat: add runner verify script for app image build

[0vertake]

Summary

• Add a runner-friendly SuperPlane app image verification script that performs required codegen and runs a full linux/amd64 Docker image build without pushing to GHCR.
• Add build-only support to release/superplane-image/build.sh via PUSH=0, preserving the existing release push behavior by default.
• Keep daily scheduling and production image publishing out of scope for this PR; those will be handled separately.

Test Plan

• Successfully ran the build through a SuperPlane runner on AWS.
• Instance: t3.xlarge
• Disk: 30GB
• Codegen: protobuf, OpenAPI spec, OpenAPI Go client, and OpenAPI TypeScript client all passed.
• Image produced locally on the runner: superplane:runner-verify
• Image size: 399MB
• Clean run disk usage: approximately 12GB
• Confirmed no GHCR push is performed by the verification path.

Closes #4693

[superplanehq-integration]

👋 Commands for maintainers:

• /sp start - Start an ephemeral machine (takes ~30s)
• /sp stop - Stop a running machine (auto-executed on pr close)

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