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/meta/_journal.json

@@ -113,6 +113,13 @@
       "when": 1771865100000,
       "tag": "0015_project_color_archived",
       "breakpoints": true
+    },
+    {
+      "idx": 16,
+      "version": "7",
+      "when": 1771955900000,
+      "tag": "0016_agent_icon",
+      "breakpoints": true
     }
   ]
 }