Fix Issue Triage workflow Copilot auth by using GitHub Actions token

[Copilot]

The Issue Triage workflow was failing in the agent job with repeated Copilot provider 401 errors, preventing triage execution. This change updates token wiring in the locked workflow so Copilot CLI auth uses the workflow-scoped GitHub token consistently.

• Auth source normalization

  • Replaced secrets.COPILOT_GITHUB_TOKEN with github.token for COPILOT_GITHUB_TOKEN in all relevant execution paths of .github/workflows/issue-triage.lock.yml.

• Updated locations

  • Secret validation step environment.
  • Main agent execution environment.
  • Detection execution environment.

• Net effect

  • Removes dependency on a separately managed repo secret for this workflow path and aligns Copilot auth with the token guaranteed in the run context.

env:
  COPILOT_GITHUB_TOKEN: ${{ github.token }}