fix(skills): harden official skill remote install

[GatewayJ]

Summary

• add official Hub remote skill install support through the backend API and workspace upload dialog
• paginate AgenticHub skill tree traversal by cursor and bound remote archive traversal
• remove stale remote preview usage, filter invalid official skill rows, and lazy-load remote skills only when the remote install tab is visible

Validation

• env GOCACHE=/tmp/gocache go test ./internal/api ./internal/skill/...
• pnpm --dir web/app typecheck
• pnpm --dir web/app test tests/api/skills.test.ts tests/models/skillhub.test.ts tests/components/HubDetailPane.test.tsx tests/components/WorkspaceTabPanels.test.tsx tests/hooks/useWorkspaceShellController.test.tsx tests/shared/i18n.test.ts
• pnpm --dir web/app lint
• git diff --check

Notes

• frontend checks still report the existing Node engine warning because this machine is using Node v26.0.0 while the app declares >=22.13.0 <25