Add rate limits example

[beanscg]

Closes #18.

Summary

• add a standalone examples/rate-limits example with helio.yaml, README, package metadata, and start script
• demonstrate a per-tool sliding-window limit for get_weather
• include the accepted key: agent config shape for send_email, with a README/config note that current MCP requests do not carry agent identity and Helio falls back to the tool bucket at runtime
• add the rate-limits example to the root README examples list

Verification

• npx --yes pnpm@10.11.0 exec prettier --check README.md examples/rate-limits/README.md examples/rate-limits/helio.yaml examples/rate-limits/package.json examples/rate-limits/start.mjs
• npx --yes pnpm@10.11.0 --filter @gethelio/proxy build
• node packages/proxy/dist/cli.js validate -c examples/rate-limits/helio.yaml
• node examples/rate-limits/start.mjs
• local smoke: called get_weather four times through http://localhost:3000/mcp; the fourth call returned reason: "rate_limited" from rule limit-weather-lookups

Note: the config validator emits the expected warning for key: agent because the current runtime falls back to key: tool until MCP requests carry agent identity.