feat: enrich installed skills links and audits metadata

[GcsSloop]

Summary

• show gray account status when no usage driver configured
• add backend enriched installed skills endpoint with catalog/audit/source links
• add frontend installed skills enrichment cache and UI entries for source + skills.sh + audits

Testing

• go test ./internal/api -run 'TestToolingHandlerListsInstalledSkillsEnrichedWithCatalogLinks|TestToolingHandlerCanInstallDiscoveredSkillWithSkillsPrefixFallback' -v
• npm --prefix frontend run test -- ToolingPage.test.tsx AccountsPage.test.tsx