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/migrations/0016_agent_icon.sql

@@ -0,0 +1 @@
+ALTER TABLE "agents" ADD COLUMN "icon" text;