Add agenttrace to observability

[luoyuctl]

Summary

• Add agenttrace to the Observability framework list.

Why

• It gives AI engineers a local-first way to inspect coding-agent logs, token usage, cost, latency, and session health.

Validation

• git diff --check README.md

Disclosure: I maintain agenttrace.

[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.