Enable BYOK Copilot by default with explicit false opt-out

[Copilot]

Summary

Enable byok-copilot by default and allow disabling it explicitly with features.byok-copilot: false.

Changes

• Updated isFeatureEnabled to treat byok-copilot as enabled by default when not explicitly set.
• Preserved explicit opt-out behavior via frontmatter boolean false (including case-insensitive feature key matching already supported by existing logic).
• Added/updated workflow tests for:
  • default-on BYOK behavior
  • explicit disable behavior
  • implicit cli-proxy implications under default-on BYOK
  • copilot installer/model fallback expectations under default-on BYOK
• Updated docs to reflect new default and opt-out behavior:
  • /home/runner/work/gh-aw/gh-aw/docs/src/content/docs/reference/frontmatter.md
  • /home/runner/work/gh-aw/gh-aw/docs/src/content/docs/reference/glossary.md

Validation

• Ran selective tests for affected workflow and Copilot areas (all passing):
  • go test -v -run 'TestIsFeatureEnabled|TestCopilotInstallerByokFeatureUsesLatestVersion|TestCopilotFallbackModelMapsToNativeEnvVar|TestCopilotEngineByokFeatureSetsDummyAPIKey|TestBuildAWFArgsCLIProxy' ./pkg/workflow/
  • go test -v -run 'TestCopilot' ./pkg/workflow/
  • go test -v -run 'TestCopilotGitHubRemotePersonalAccessTokenExport|TestCopilotDetectionDefaultModel' ./pkg/workflow/
• Ran make agent-finish before commit; existing unrelated timeout persists in pkg/cli test suite (FAIL github.com/github/gh-aw/pkg/cli 600s) consistent with baseline run.
• Ran parallel_validation:
  • Code Review: no comments
  • CodeQL: 0 alerts (go analysis skipped due oversized database)