Automate testnode image publishing on tags

[douglance]

Summary

• publish testnode images automatically when a v* tag is pushed
• use the default testnode config for tag-triggered publishes, with the tag as the image version
• document the automatic tag publish flow and correct the image repository in the README
• add resolver coverage for tag-provided versions and explicit input overrides

Validation

• actionlint .github/workflows/release-testnode-image.yml
• pnpm build
• pnpm typecheck
• pnpm test:run
• pnpm exec biome check test/resolve-testnode.test.ts README.md

Notes

• pnpm lint still fails on the existing packages/core/src/init/runner.ts cognitive-complexity warning on origin/main; this PR does not touch that file.