feat(web_search): add load balance and failover for web api keys

[yanhool]

Summary

Add load balance and failover support for web api keys.

Problem

Free-tier web search APIs enforce usage limits, which restrict high-volume workloads. Multi-agent scenarios may also trigger rate limits.

Solution

Introduce an atomic, thread-safe rotating key pool to enable load balance and failover.

Changes

• pkg/agent/loop.go, pkg/config/config.go, pkg/config/defaults.go, pkg/migrate/config.go: rename variables
• pkg/tools/web.go: implement load balance and failover logic

Usage

"tools": {
  "web": {
    "brave": {
      "enabled": true,
      "api_key": "brave-key-1, brave-key-2, brave-key-3",
      "max_results": 5
    }
  }
}