Docs: add multi-agent failure map (ProblemMap No.13) and link from README (#23)

[onestardao]

Closes #23

Summary

This PR adds a docs-only troubleshooting section for multi-agent setups in LightAgent / LightSwarm, aligned with ProblemMap No.13 (multi-agent chaos).

Changes

1. New doc: docs/multi_agent_failure_map.md

   • Includes a concise 16-problem overview.
   • Focuses on No.7 (memory fracture) and No.13 (multi-agent chaos).
   • Provides a symptom → mode → what-to-inspect checklist for LightAgent users.

2. README update

   • Adds a "Multi-agent troubleshooting" section under Features.
   • Links to the new failure map page.

Scope

• Docs only.
• No changes to core framework code or APIs.
• Diagnostic guidance only (no architectural enforcement).

The failure-mode vocabulary is adapted from an MIT-licensed open-source 16-problem ProblemMap for RAG and agentic pipelines.

[wxai-space]

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