feat: add streaming output, model selection with dropdown, and fallback

[Dhairya0707]

What does this PR do?

This PR introduces streaming output and an interactive, real-time dropdown selection menu for choosing models within the CLI.

Streaming output:

• Responses are printed token-by-token as they're generated from Gemini, instead of waiting for the full response.
• Tool-calling responses are intelligently detected mid-stream and suppressed, then the final answer streams live.

Interactive model selection:

• Intercepts keystrokes in real-time. As soon as the user types /model, the CLI clears the prompt and transitions into an interactive select mode.
• Allows navigating available models using the standard Up (↑) and Down (↓) arrow keys.
• Confirms the model change instantly upon pressing Enter, and restores the regular prompt.
• Gracefully cancels selection and returns to the prompt upon pressing Escape or Ctrl+C.
• Retains backward compatibility (you can still hit enter on /model to trigger the dropdown, or type /model <model-name> directly).
• Falls back to the default model automatically if the selected model fails.

Type of change

• Bug fix
• New feature / tool
• Documentation update
• Refactor

How to test it

1. Start the CLI application:

   node src/index.js

2. Type hello and observe the response stream in token-by-token.
3. Type /model into the prompt and observe the select menu:

   Agent > Select a model:
     ➔  gemini-3.1-flash-lite
        gemini-2.5-pro
        ...
   

4. Use the Up/Down Arrow Keys to navigate through the models.
5. Press Enter to select the highlighted model. The CLI will confirm the switch and return you to the main prompt.
6. Trigger the menu again and press Escape to verify it cancels gracefully without switching models.
7. Type list files and observe the tool execute with the final answer streaming.

Related issues

Closes #21

[HoussemEddineChaouch]

Hi @Dhairya0707! Thanks for the contribution.

The merge conflicts appeared after PR #34 was merged into main. Could you please resolve the conflicts, and push the updated branch?

Thanks!

[vercel]

@Dhairya0707 is attempting to deploy a commit to the houssemeddinechaouch's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Dhairya0707]

Done it checkout it

[abusnitsky]

Why remove messages = []?

[abusnitsky]

But still use it? Did you test this after fixing the conflicts?

[abusnitsky]

Wouldn't it be better to break this down into three separate, simpler issues? Right now, it's a bit hard to understand and review all at once.

[Dhairya0707]

Sure Just 10 min

[Dhairya0707]

I've split this into 2 cleaner PRs for easier review:

• feat: add streaming output with fallback #38 — Streaming output + model env var + fallback
• feat: add interactive model selection dropdown #39 — Interactive model selection dropdown + slash commands