Fix: banner on launch, config file loading, 7 providers match setup wizard

BUGFIXES:
- Banner now displays on 'gyandoot run' via gyandoot_tui::banner::print_banner()
- Config now loads from ~/.config/gyandoot/config.toml with fallback chain
  (XDG -> ~/.gyandoot/config.yaml -> hardcoded defaults), fixing 401 errors
- gyandoot init now writes visible placeholder api_key for users to edit

NEW PROVIDERS (4 added, now 7 total matching setup wizard):
- Google Gemini: Native API with x-goog-api-key auth, streamingGenerateContent
- DeepSeek: OpenAI-compatible wrapper at api.deepseek.com/v1
- OpenRouter: With HTTP-Referer/X-Title headers, model routing
- Groq: OpenAI-compatible wrapper at api.groq.com/openai/v1

All compile with 0 errors.

Cargo.toml

@@ -80,4 +80,5 @@ clap.workspace = true
 anyhow.workspace = true
 tracing.workspace = true
 tracing-subscriber.workspace = true
+serde_yaml.workspace = true
 dirs.workspace = true