Restore existing-project default command routing in Effect CLI

[eluce2]

Summary

The rewritten @proofkit/cli now ships the public proofkit binary, publishes from the new package, and exposes the migrated command surface through Effect CLI. However, the root no-arg command inside an existing ProofKit project still shows explicit command guidance instead of routing into the interactive project-management flow.

Follow-up

• Decide whether the existing project root command should open an Effect-native menu or adapt the legacy menu flow.
• Preserve the current no-arg scaffold routing when no proofkit.json is present.
• Add CLI/runtime coverage for the existing-project default command path.

Why

This is the main remaining UX gap after the CLI package rename/migration work landed.