Add GitHub Copilot ACP runtime

[jsgrrchg]

Summary

• add GitHub Copilot CLI as an external ACP runtime using copilot --acp
• add safe integrated copilot login setup without persisting GitHub credentials
• preserve Copilot ACP modes, permissions, review diffs, diagnostics, and documentation

Validation

• cargo test -p neverwrite-native-backend ai
• targeted Vitest AI provider, controls, and store tests
• npm run lint
• npm run build
• npm run electron:sidecar:build
• npm run electron:ai-runtime:smoke

Notes

• Copilot models remain runtime-discovered through ACP; the current CLI advertises Auto rather than a model catalog. I can't test this because the cli only reports auto as model, I need help from Copilot subscribers to test the model selector.