feat: add username/slug based agent API endpoints

- Add /api/[username]/agents/[agentSlug]/conversation/[conversationId]
- Add /api/[username]/agents/[agentSlug]/conversations
- Update agent dashboard API docs to use new URL format
- Include user.username in agent API response
- Update Discord summary workflow to use new endpoint format

.github/workflows/discord-summary.yml

@@ -26,10 +26,11 @@ jobs:
           echo "Triggering agent with conversation: ${{ steps.conv-id.outputs.conv_id }}"
 
           # POST to the agent conversation endpoint
-          # Agent ID: Discord Daily Summary agent on tpmjs.com
+          # Uses username/agent-slug URL format: /api/{username}/agents/{agent-slug}/conversation/{conv-id}
+          # Agent: ajax/tpmjs-discord-summary
           # Requires API key with agent:chat scope
           RESPONSE=$(curl -s -X POST \
-            "https://tpmjs.com/api/agents/cmk9utpvg0001kz04csbisvf5/conversation/${{ steps.conv-id.outputs.conv_id }}" \
+            "https://tpmjs.com/api/ajax/agents/tpmjs-discord-summary/conversation/${{ steps.conv-id.outputs.conv_id }}" \
             -H "Authorization: Bearer $TPMJS_API_KEY" \
             -H "Content-Type: application/json" \
             -d '{