fix: preserve trusted gh authentication for GitHub adapter

[sfloess]

Summary

• preserve GH_TOKEN/GITHUB_TOKEN for the trusted gh subprocess
• keep credentials outside agent state and worker results
• add regression coverage for CI authentication semantics
• document the GitHub dogfood acceptance boundary

Why

The previous adapter explicitly removed GH_TOKEN and GITHUB_TOKEN before invoking gh. That prevents the normal GitHub Actions authentication path from working. Credentials should remain outside worker state, but the trusted publication boundary must still be able to consume credentials supplied by its parent environment.

Tests mock gh; no real credentials are required.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud