Assign conversations to agents based on SLA priority

Problem
Currently there's no way to automatically assign conversations to agents based on their SLA urgency, so high-priority messages with tight SLAs may be routed the same way as lower-priority items with longer SLAs.

Request
Add an assignment option that routes conversations to agents in order of SLA priority, so faster-SLA items (e.g., chat with 1-hour SLA) reach higher-level agents before slower-SLA items (e.g., email with 2-hour SLA).