Add A3M Router (#1 on RouterArena) to Agent infrastructure

[Das-rebel]

What's Added

A3M Router in the Agent infrastructure section, alongside claude-code-router.

Why A3M belongs here

• LLM router for CLI agents — routes queries to the cheapest capable model across 47+ providers
• 🥇 Add pi-builder to Agent infrastructure #1 on RouterArena (76.43) — the official standardized benchmark for LLM routers
• Cheapest at $0.047/1K queries — 4x cheaper than nearest competitor
• npx a3m-router route "your query" — drop-in for any CLI agent that needs model routing
• Open-source, 19.5KB, zero ML deps

Benchmark: RouteWorks/RouterArena#113
GitHub: https://github.com/Das-rebel/a3m-router

[Das-rebel]

Hi! Following up on this PR. A3M Router has since achieved #1 on the RouterArena benchmark (76.43 score). It's a CLI tool that routes LLM queries — fits the Agent infrastructure section well. Let me know if any changes are needed!