feat(agent): add llm assistant chat with api tools (task 88)

OpenAI-compatible client with mock fallback, tool-calling loop, and /assistant UI.

Tools map to /api/v1/ endpoints per ADR 0006 direct-tools decision.

src/components/nav-icon.tsx

@@ -21,6 +21,7 @@ import {
   Lock,
   Mail,
   Menu,
+  MessageCircle,
   MoreHorizontal,
   Pencil,
   Phone,
@@ -74,6 +75,7 @@ const ICONS: Record<string, React.ComponentType<LucideProps>> = {
   sprout: Sprout,
   mail: Mail,
   menu: Menu,
+  "message-circle": MessageCircle,
   more: MoreHorizontal,
   x: X,
   "chevron-left": ChevronLeft,