[runtime] remove redundant keygen helper, add Dockerfile, clean smoke test

[GsCommand]

Motivation

• Consolidate runtime tooling onto the canonical key generator, provide a simple container entrypoint for the runtime service, and remove a dead smoke-test helper that encoded a stale fixed-kid assumption.

Description

• Delete the redundant gen-ed25519.mjs and retain tools/mkkeys.mjs as the canonical keygen utility used by scripts/dev.sh and the README.
• Add a minimal runtime Dockerfile based on node:20-bookworm-slim that runs npm ci --omit=dev, copies server.mjs, exposes port 8080, and starts the app with npm start.
• Remove the unused assertReceiptInvariants() helper from tests/smoke.mjs so the smoke test no longer hardcodes kid === "v1" and instead relies on the active extractProof() assertions that validate the runtime's proof shape.

Testing

• Ran npm run check (Node syntax check) which completed successfully.
• Ran the smoke script with node tests/smoke.mjs, which completed successfully (smoke ok).
• No other automated tests were modified.

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Codex Task