fix: /login shows provider menu and supports Ctrl+C cancellation

[assapir]

Summary

Fixes #54 — /login now lets you choose a provider and can be cancelled with Ctrl+C.

Changes

1. Provider selection menu (src/commands/login.rs)

/login now presents a numbered menu of all available providers (like /model does for models), instead of always logging into the current provider:

  Choose a provider:

  1. Anthropic (Claude Pro/Max) ← current
  2. Google (Gemini)

  Select provider: 

The current provider is marked. Auth status is only updated in the REPL when logging into the current provider.

2. Async stdin for Anthropic login (src/provider.rs)

Switched from blocking io::stdin().read_line() to async tokio::io::stdin().lines() so the "Paste the authorization code" prompt is cancellable by the tokio runtime.

3. Ctrl+C support for all commands (src/main.rs)

Wrapped commands.dispatch() in tokio::select! with tokio::signal::ctrl_c(), so any interactive command (not just task execution) can be interrupted. This covers:

• The provider selection prompt
• Anthropic's code-paste wait
• Google's OAuth callback wait

4. New all_login_providers() helper (src/provider.rs)

Returns all providers that support login, used by the /login menu.

Testing

All 214 tests pass. Zero clippy warnings. cargo fmt clean.

[Copilot]

Pull request overview

This PR improves the REPL login experience by adding an interactive provider-selection menu to /login and making interactive command flows cancellable via Ctrl+C, aligning /login with existing menu-driven UX patterns in the REPL.

Changes:

• Add a provider selection menu to /login, with current provider marking and conditional auth-status updates.
• Make Anthropic’s “paste authorization code” prompt async to support cancellation.
• Wrap command dispatch in tokio::select! with tokio::signal::ctrl_c() to allow interrupting interactive commands.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/provider.rs	Switch Anthropic login code entry to async stdin and add all_login_providers() helper.
src/main.rs	Add Ctrl+C handling around command dispatch to interrupt interactive commands.
src/commands/login.rs	Implement provider selection menu and async stdin input for /login.

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