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/shared/src/validators/agent.ts

@@ -27,6 +27,7 @@ export const createAgentSchema = z.object({
   name: z.string().min(1),
   role: z.enum(AGENT_ROLES).optional().default("general"),
   title: z.string().optional().nullable(),
+  icon: z.string().optional().nullable(),
   reportsTo: z.string().uuid().optional().nullable(),
   capabilities: z.string().optional().nullable(),
   adapterType: z.enum(AGENT_ADAPTER_TYPES).optional().default("process"),