Add agenttrace observability tool

[luoyuctl]

Summary

• Add agenttrace to the Observability section in 05_resources/stack-tools.md
• Describe its local TUI/reporting focus for AI coding agent session cost, token, latency, and failure inspection

Validation

• git diff --check
• No project-level build or test entrypoint found for this Markdown-only resource update

[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.