Mention agent swarm branch in README

[OnePunchMonk]

Summary

• Add a README section calling out the experimental agent-swarm-method branch.
• Summarize the swarm architecture: memory agent, regime analyst, strategy specialists, critic agent, and multi-window backtest coordinator.
• Include commands for readers to inspect the branch and run its swarm tests.

Validation

• Ran git diff --check for Markdown whitespace validation.

Notes

• This is documentation-only and describes the swarm implementation as experimental rather than part of the main ReAct pipeline.