feat: Add Forge as LLM provider

[Yiiii0]

Description

Adds Forge as an OpenAI-compatible gateway provider in nanobot's provider registry, following the same pattern as AiHubMix.

Changes Made

• nanobot/nanobot/providers/registry.py: Added ProviderSpec for Forge with is_gateway=True, litellm_prefix="openai", strip_model_prefix=True
• nanobot/nanobot/config/schema.py: Added forge field to ProvidersConfig
• nanobot/nanobot/providers/litellm_provider.py: Extended env_extras handling to use os.environ[...] (override) for gateways instead of setdefault, matching the primary env_key pattern at line 60
• nanobot/README.md: Added Forge to provider table

Usage

# In nanobot config:
providers:
  forge:
    api_key: "your-forge-api-key"
    api_base: "https://api.forge.tensorblock.co/v1"  # optional, this is the default

Checklist

• Changes tested locally
• Documentation updated
• Ran pre-commit hooks

I work at TensorBlock and will help maintain this integration.

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About Forge

Forge is an open-source middleware service for unified AI model provider management. It routes requests across 40+ AI providers with access to thousands of models through a single OpenAI-compatible API.

• Repo: https://github.com/TensorBlock/forge
• Docs: https://www.tensorblock.co/api-docs/overview