Fix Issue Monster agent assignment routing

[Copilot]

Issue Monster was running on schedule but not reliably scheduling work: recent runs commented on selected issues without producing assign_to_agent assignments. The workflow also needed to route assignments through the intended custom agent and Propelit model.

• Assignment defaults
  • Configure assign-to-agent to use Copilot with the agentic-workflows custom agent.
  • Pass propelit as the assignment model.

safe-outputs:
  assign-to-agent:
    name: copilot
    model: propelit
    custom-agent: agentic-workflows

• Prompt behavior

  • Clarify that Issue Monster must call assign_to_agent before posting the selected-for-Copilot comment.
  • Align the mission text with the actual 30-minute cadence.

• Evaluation alignment

  • Update the assignment eval to match the workflow’s configured maximum of three assignments per run.