Feature: add ForgeCode agent solver

[hmyuuu]

Summary

Added ForgeCode as a Harbor solver with provider/model and reasoning-effort selection.

Validation

harbor run -p tasks/challenge-01 \
  -a adapters.forgecode:ForgeCode \
  -m codex/gpt-5.5 \
 --agent-kwarg codex_auth_json_path= <auto.json path> \
  --ak reasoning_effort=xhigh \
  -e adapters.framework_docker:FrameworkDockerEnvironment \
  --cpus ignore --memory ignore