Add configure connection delete command

[ewega]

Summary

Add a configure connection delete subcommand to remove connections that are broken, test connections, or no longer needed.

Design (Option A: under configure connection)

gh devlake configure connection delete # interactive gh devlake configure connection delete --plugin gh-copilot --id 3 # flag-based

Interactive flow:

1. List all connections across plugins
2. User selects one to delete
3. Confirmation prompt with warning about losing scopes
4. DELETE /plugins/{plugin}/connections/{id}
5. Update state file

Changes

• Add delete subcommand under configure connection
• Interactive: list connections, prompt select, confirm
• Flag-based: --plugin + --id required
• Call DELETE /plugins/{plugin}/connections/{id}
• Remove from state file after successful deletion
• Add DeleteConnection method to devlake.Client

Acceptance Criteria

• Interactive mode lists connections and prompts for confirmation
• Flag mode works with --plugin + --id
• State file updated after deletion
• Confirmation prompt warns about scope deletion
• Graceful error if connection ID doesn't exist