Stabilize Copilot Actions job by pinning a supported cloud-agent model

[Copilot]

The copilot GitHub Actions job was failing during agent startup because the resolved model (sweagent-capi:claude-sonnet-4.6) is no longer available. This change ensures Copilot sessions start with a supported model in this repository.

• Root cause

  • Copilot runtime resolved to an unavailable model and aborted before task execution (Model "... is not available").

• Change

  • Added a dedicated Copilot setup workflow at .github/workflows/copilot-setup-steps.yml.
  • Introduced a setup step that exports a supported model for Copilot cloud agent sessions.

• Effective behavior

  • Copilot jobs now initialize with a valid model instead of inheriting an unavailable default.

jobs:
  copilot-setup-steps:
    steps:
      - name: Force supported Copilot model
        run: echo "COPILOT_AGENT_MODEL=sweagent-capi:claude-sonnet-4.5" >> "$GITHUB_ENV"