feat(instances): import local standalone nullclaw homes

[vernonstinebaker]

Summary

• add support for importing local same-machine NullClaw standalone homes from either the default ~/.nullclaw path or a custom path
• add standalone detection and imported-runtime handling so attached custom-path installs report correctly in NullHub
• add the install-page UI flow for attaching an existing NullClaw with path and optional name

Testing

• zig build test --summary all
• zig build
• npm run build

Notes

• scope is limited to local same-machine NullClaw imports
• imported external instances are expected to follow the normal log convention: logs/stdout.log 2>&1
• no automated UI test harness was added in this change