[User Story] As a user, I want `git-copilot config` to let me add/remove/list LLM providers interactively

[coderabbitai]

User Story

As a developer setting up git-copilot for the first time,
I want to run git-copilot config --add openai and be prompted for my API key,
So that I can configure the tool without manually editing JSON files.

Acceptance Criteria

• git-copilot config --list prints all configured providers and marks the active one
• git-copilot config --add <name> prompts for baseUrl, apiKey, model, maxTokens via inquirer
• git-copilot config --remove <name> removes the provider after confirmation
• git-copilot config --set-active <name> switches the active provider
• API key is stored securely (see linked security issue)
• Configuration changes are validated before saving

Notes

inquirer is already a listed dependency. ConfigManager already has addProvider, removeProvider, setActiveProvider methods. Wiring the CLI options to these methods is the primary work.

Backlink: #1