Fix GitHub Copilot branding

[bkallday1122]

Summary

• normalize two text-only references from Github Copilot to GitHub Copilot
• keep scope limited to one config comment and one user-facing connection error

Validation

• git diff --check
• rg -n "Github Copilot|GitHub Copilot" pkg/config/config.go pkg/providers/github_copilot_provider.go
• go test ./pkg/config ./pkg/providers could not be run because go is not installed in this environment

[508704820]

Code review — Approve.