feat: add Gemini CLI auth login support

[AtharvaGurao]

Summary

Adds Gemini (Google AI) as a supported provider for CLI authentication, enabling users to securely store and use their Gemini API key.

Usage

# Login (paste API key from aistudio.google.com/apikey)
picoclaw auth login --provider gemini

# Check status
picoclaw auth status

# Logout
picoclaw auth logout --provider gemini

The API key is stored securely in ~/.picoclaw/auth.json and loaded automatically when the provider is configured with auth_method: "token".

Changes

File	Change
pkg/auth/token.go	Added Gemini display name (aistudio.google.com/apikey)
cmd/picoclaw/main.go	Wired Gemini into login, logout, help commands
pkg/providers/http_provider.go	Load Gemini API key from auth store when auth_method is token (both explicit provider and model-name fallback)

How it works

1. User runs picoclaw auth login --provider gemini
2. Prompted to paste API key from Google AI Studio
3. Key is stored in ~/.picoclaw/auth.json
4. Config is updated with auth_method: "token"
5. On next run, the Gemini provider loads the key from the auth store

Backward Compatible

• Existing api_key in config continues to work (takes priority)
• Only 3 files changed, 38 insertions, 8 deletions

Closes #267

[Leeaandrob]

@Zepan Adds Gemini CLI auth login support — allows using Google's device auth flow for Gemini access without an API key.

Recommendation: Merge. +38/-8, lowers the barrier to using Gemini. Aligns with PicoClaw's goal of being accessible on minimal hardware with minimal setup.