feat: set GITHUB_COPILOT_INTEGRATION_ID env var for Copilot CLI

[eaftan]

When running the Copilot CLI in agentic workflows, set GITHUB_COPILOT_INTEGRATION_ID=agentic-workflows so the CLI knows it is being invoked by gh-aw.

Changes

• Added CopilotCLIIntegrationIDEnvVar and CopilotCLIIntegrationIDValue constants in pkg/constants/constants.go
• Set the env var in pkg/workflow/copilot_engine_execution.go for all Copilot engine execution steps
• Updated golden test files and recompiled all workflow lock files

[github-actions]

Hey @eaftan 👋 — great work on the Copilot CLI integration ID change! Setting GITHUB_COPILOT_INTEGRATION_ID=agentic-workflows so the CLI can identify its caller is a clean, well-scoped improvement, and the implementation follows the existing patterns in pkg/constants/engine_constants.go and pkg/workflow/copilot_engine_execution.go nicely.

Everything here looks solid:

• ✅ Clear description of what changed and why
• ✅ Constants properly defined with doc comments in engine_constants.go
• ✅ Golden test files (basic-copilot.golden, with-imports.golden) updated to reflect the new env var
• ✅ All 119 lock files recompiled consistently
• ✅ No new dependencies introduced

This PR looks ready for maintainer review whenever you're happy to mark it out of draft! 🚀

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[pelikhan]

This will get stale. Will reopen.