Surface SailApps confirmation code during OAuth login

[luke-hagar-sp]

Summary

• Prints the SailApps confirmation code during OAuth login so users can verify the browser approval page matches the CLI session.
• Derives the code from the auth session UUID using the same last-eight-character, XXXX-XXXX format shown by the SailApps page.
• Adds unit coverage for confirmation code formatting edge cases.

Test plan

• GOWORK=off go test -count=1 ./internal/config
• GOWORK=off go build ./...

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