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.

[nikolasdehor]

Well-structured change that follows existing patterns for OpenAI and Anthropic auth. The CLI integration is complete (login, logout, help, status) and the provider loading correctly checks both APIKey and AuthMethod.

Like PR #333, this needs a rebase against current main because http_provider.go's CreateProvider() has been refactored into factory.go. The auth credential loading logic needs to be ported to the new factory pattern.

The providerDisplayName returning "aistudio.google.com/apikey" is a nice touch for guiding users. Once rebased, LGTM.

[CLAassistant]

All committers have signed the CLA.