Add npm support metadata

[yanziwei]

Summary

• add npm homepage and bugs.url metadata for the root skilld package
• add npm homepage and bugs.url metadata for skilld-protocol

This is metadata-only. Runtime code, dependencies, exports, package versions, and build output are unchanged.

Validation

node manifest assertion for package.json and packages/protocol/package.json
git diff --check
(cd packages/protocol && npm pack --dry-run --ignore-scripts --json)

The dry-run package includes package.json.