FC: add assignment preset for controller on :5020 + clearer diagnostics

[mkimhi]

Addresses #211.

Problem: When users start the AgentBench FC stack via docker compose -f extra/docker-compose.yml up, the AgentRL controller runs on port 5020 (host network). However, the default assignment definition points TaskClient to http://localhost:5000/api, so src.assigner cannot see any workers and appears to make no progress.

Changes:

• Add configs/assignments/definition_fc.yaml + configs/assignments/default_fc.yaml that point controller_address to http://localhost:5020/api.
• README: document running assigner with configs/assignments/default_fc.yaml for the FC docker-compose stack.
• Improve the TaskClient connection warning to include the configured controller URL and a hint about the 5020 default for the FC stack.

No benchmark logic changes; this is config + docs + clearer diagnostics.