feat(agent): add Claude Code-style agent, skills, slash-commands, hooks, todos, sandboxed workspace, and full-stack scaffolding

Adds Claude Code-inspired agent capabilities:
- Agent loop with tools (read/write/edit/glob/grep/bash/todos)
- Skills system (markdown skill files, runtime loading)
- Slash commands (/commit /review /feature /design /explain /test /refactor /skill /help)
- Hooks system (block-dangerous-rm, warn-debug-code, warn-secrets-in-code, warn-eval-exec)
- Sandboxed workspace with path-escape protection
- Todo lists (todo_write / todo_read / todo_update)
- Full-stack scaffolding skill
- No external SDK dependencies; no Next.js conversion

index.html

@@ -672,6 +672,9 @@ body.hide-thinking .think-block { display: none; }
 .tab-pane { display: none; height: 100%; min-height: 0; }
 .tab-pane.active { display: flex; flex-direction: column; }
 
+/* Agent tab — scrollable like the Deploy tab */
+#pane-agent { padding: 16px; gap: 14px; overflow-y: auto; }
+
 /* Preview tab */
 #pane-preview {
   align-items: stretch; justify-content: stretch;