Add GitHub Action trace report

[ding-swj]

Body:
Add a GitHub Action integration that can publish or link an AgentScope trace report for pull requests.

Scope:

• Define expected action inputs
• Generate or consume a trace JSON artifact
• Add a PR comment with trace summary and viewer link
• Document setup in README

Acceptance criteria:

• Example workflow is documented
• PR comment includes status, trust score, changed files, and commands
• Trace artifact can be opened in AgentScope