KAP-945: fix autopilot update agent reassignment

[milymarkovic]

Summary

• send assignee_type=agent with assignee_id for autopilot update --agent
• add regression coverage for squad-to-agent reassignment using a fixture server

Test plan

• go test ./cmd/multica -run TestRunAutopilotUpdateAgentSwitchesAssigneeType -count=1
• compiled package test binary run outside Multica daemon marker: /tmp/kap945-multica.test -test.run Autopilot -test.count=1

Closes KAP-945

[vercel]

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