Allow strict negotiate workflow with 2+ agents

[justinmoon]

Summary

• allow strict negotiate sessions with two or more agents
• keep the strict challenge gate but scale required cross-agent challenges by participant count
• add tests covering 2-agent and 3-agent agree-threshold behavior

Testing

• cargo test --manifest-path /Users/justin/code/rally/worktrees/negotiate-workflow/Cargo.toml -p rally-workflow-plan