Fix: update HandoffBuilder API call from set_coordinator to with_start_agent

The agent-framework package 1.0.0b260114 renamed the HandoffBuilder method
from 'set_coordinator' to 'with_start_agent'. This change ensures compatibility
with the latest version of the package.

Author: hunterjam

content-gen/src/backend/orchestrator.py

@@ -443,7 +443,7 @@ def initialize(self) -> None:
                 image_content_agent,
                 compliance_agent,
             ])
-            .set_coordinator(triage_agent)
+            .with_start_agent(triage_agent)
             # Triage can hand off to all specialists
             .add_handoff(triage_agent, [
                 planning_agent,