feat: add agent icons with picker and collapsible sidebar section

- Add `icon` text column to agents DB schema with migration
- Add icon field to shared Agent type and validators
- Create AgentIconPicker component with 40+ curated lucide icons and search
- Show clickable icon next to agent name on detail page header
- Replace static Agents nav item with collapsible AGENTS section in sidebar
- Each agent shows its icon (defaulting to Bot) with truncated name

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/db/src/schema/agents.ts

@@ -18,6 +18,7 @@ export const agents = pgTable(
     name: text("name").notNull(),
     role: text("role").notNull().default("general"),
     title: text("title"),
+    icon: text("icon"),
     status: text("status").notNull().default("idle"),
     reportsTo: uuid("reports_to").references((): AnyPgColumn => agents.id),
     capabilities: text("capabilities"),