Add agenttrace to evaluation tools

[luoyuctl]

Summary

• Add agenttrace to the Observability & Monitoring section in README.
• Add a concise agenttrace entry to tools-and-platforms.md with features, use cases, and install command.

Checks

• git diff --check

This is a docs-only addition.

[luoyuctl]

Refreshed this PR by resetting the commit author to the GitHub noreply identity. Validation: git diff --check upstream/main...HEAD. Current PR state is CLEAN/MERGEABLE with no checks configured.