Add --plugin flag to scope/project/full commands

[ewega]

Summary

Replace --skip-copilot/--skip-github\ with a positive --plugin\ flag. One plugin per invocation in flag mode; interactive mode walks through plugins sequentially.

Design

• --plugin github\ or --plugin gh-copilot\ — configure only that plugin
• Omitted --plugin\ in flag mode: error asking user to specify
• Interactive mode (no flags): prompt to select plugin, loop for additional
• \configure full: interactive multi-plugin orchestration remains, --plugin\ limits to one
• Deprecate --skip-copilot, --skip-github\ (keep as hidden flags for one release)

Future-proofing

This scales to N plugins without adding per-plugin skip flags. When GitLab/Azure DevOps/Jenkins arrive, they just register in \connectionRegistry\ and show up in the interactive picker.

Acceptance Criteria

• --plugin\ flag works on scope, project, and full commands
• Omitting --plugin\ in flag mode produces clear error
• Interactive mode prompts for plugin selection
• Old skip flags still work but are hidden from help