Modularize testnode workspace and publish snapshot-backed variants

[douglance]

Summary

• Move the CLI into apps/cli and split reusable code into workspace packages.
• Replace the old compose-only start path with a published-image start workflow.
• Add snapshot-backed testnode variant publishing for l2, l3-eth, and custom gas token variants.
• Rename runtime/action concepts around the final ontology: testnode product, variant target, snapshot source, and start workflow.
• Update README and action docs for the new CLI, action, and publish flow.

Validation

• pnpm validate
• pnpm exec vitest run apps/cli/test/start.test.ts
• pnpm --filter arbitrum-testnode dev start --image-version v0.2.2 --startup-timeout-seconds 0