feat: Extension Catalog — GUI-driven extension management for sandboxes

[mafueee]

🧩 Extensions Catalog

This PR adds a GUI-driven extension management system for NemoClaw sandboxes, enabling users to browse, install, and uninstall integrations (Discord, Telegram, Slack, Docker Hub, Hugging Face, Jira, npm, PyPI, Outlook) directly from the dashboard.

What's New

Registry (nemoclaw-blueprint/extensions/registry.json)

• 9 extensions covering messaging, dev tools, registries, and productivity categories
• Each extension maps to existing policy presets and defines optional install commands

Backend API (gui/server/routes/extensions.js)

• GET /api/extensions — List all extensions with install status per sandbox
• GET /api/extensions/:id — Get extension detail
• POST /api/extensions/install — Apply policy + credentials + in-sandbox packages via ExecSandbox gRPC
• POST /api/extensions/uninstall — Remove policy and clean up

Frontend (gui/src/components/extensions/ExtensionCatalog.tsx)

• Card-based grid layout with category filtering tabs
• Sandbox selector dropdown (auto-selects first running sandbox)
• Credential input modal for token-requiring extensions
• Real-time step-by-step install/uninstall progress feedback
• Responsive design (mobile/tablet/desktop)

Integration (gui/src/App.tsx)

• New 🧩 Extensions entry in the Configuration section of the sidebar
• Route at /extensions rendering ExtensionCatalog

Tests (test/extensions.test.js)

• 9 unit tests validating registry integrity, field presence, uniqueness, preset file references, categories, and credential structure

Documentation (README.md)

• New "Extensions" section with available extensions table, "How It Works" guide, and "Adding Custom Extensions" reference
• Updated architecture description and features table

Remaining local-only changes

The following small changes need to be applied manually when merging:

gui/server/index.js — Add import and mount:

import extensionRoutes from './routes/extensions.js';
// ...
app.use(extensionRoutes);

gui/src/index.css — Add extension catalog CSS styles (approx. 500 lines of .ext-* classes at the bottom of the file)

Verification

• ✅ All 9 unit tests pass (npx vitest run test/extensions.test.js)
• ✅ GET /api/extensions returns 9 extensions with correct metadata
• ✅ Frontend builds cleanly (npm run build)