feat: add Venice AI provider integration

Venice AI is a privacy-focused AI inference provider with support for
uncensored models and access to major proprietary models via their
anonymized proxy.

This integration adds:

- Complete model catalog with 25 models:
  - 15 private models (Llama, Qwen, DeepSeek, Venice Uncensored, etc.)
  - 10 anonymized models (Claude, GPT-5.2, Gemini, Grok, Kimi, MiniMax)
- Auto-discovery from Venice API with fallback to static catalog
- VENICE_API_KEY environment variable support
- Interactive onboarding via 'venice-api-key' auth choice
- Model selection prompt showing all available Venice models
- Provider auto-registration when API key is detected
- Comprehensive documentation covering:
  - Privacy modes (private vs anonymized)
  - All 25 models with context windows and features
  - Streaming, function calling, and vision support
  - Model selection recommendations

Privacy modes:
- Private: Fully private, no logging (open-source models)
- Anonymized: Proxied through Venice (proprietary models)

Default model: venice/llama-3.3-70b (good balance of capability + privacy)
Venice API: https://api.venice.ai/api/v1 (OpenAI-compatible)

docs/providers/index.md

@@ -35,6 +35,7 @@ Looking for chat channel docs (WhatsApp/Telegram/Discord/Slack/Mattermost (plugi
 - [Z.AI](/providers/zai)
 - [GLM models](/providers/glm)
 - [MiniMax](/providers/minimax)
+- [Venice AI (privacy-focused)](/providers/venice)
 - [Ollama (local models)](/providers/ollama)
 
 ## Transcription providers