Migrate to code-mode 0.3.1 runtime driver

[ggoodman]

Summary

• replace the legacy code-mode 0.2 integration with a RuntimeDriver assembled by createRuntimeFactory
• boot one persistent Sandbox-hosted Node.js 24 runner over fd 3 using the public @torkbot/code-mode/node-runtime helpers
• keep boot cancellation, runtime disposal, and caller-owned Sandbox lifetimes distinct
• run the exported black-box testRuntime suite and focused Sandbox launch/channel/disposal tests
• update the README and public TypeScript declarations for the released ESM agent contract

Verification

• npm run typecheck
• npm test
• npm run build
• release workflow action-pin verification
• npm pack --dry-run --ignore-scripts
• external packed-consumer typecheck/import smoke, including blocked deep imports

[ggoodman]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Chef's kiss.

Reviewed commit: 2d72e84361

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