feat: add Google Gemini provider with CLI token reuse support

- Add src/providers/gemini.rs with support for:
  - Direct API key (GEMINI_API_KEY env var or config)
  - Gemini CLI OAuth token reuse (~/.gemini/oauth_creds.json)
  - GOOGLE_API_KEY environment variable fallback
- Register gemini provider in src/providers/mod.rs with aliases: gemini, google, google-gemini
- Add Gemini to onboarding wizard with:
  - Auto-detection of existing Gemini CLI credentials
  - Model selection (gemini-2.0-flash, gemini-1.5-pro, etc.)
  - API key URL and env var guidance
- Add comprehensive tests for Gemini provider
- Fix pre-existing clippy warnings in email_channel.rs and whatsapp.rs

Closes #XX (Gemini CLI token reuse feature request)

src/providers/mod.rs

@@ -1,5 +1,6 @@
 pub mod anthropic;
 pub mod compatible;
+pub mod gemini;
 pub mod ollama;
 pub mod openai;
 pub mod openrouter;
@@ -20,6 +21,9 @@ pub fn create_provider(name: &str, api_key: Option<&str>) -> anyhow::Result<Box<
         "ollama" => Ok(Box::new(ollama::OllamaProvider::new(
             api_key.filter(|k| !k.is_empty()),
         ))),
+        "gemini" | "google" | "google-gemini" => {
+            Ok(Box::new(gemini::GeminiProvider::new(api_key)))
+        }
 
         // ── OpenAI-compatible providers ──────────────────────
         "venice" => Ok(Box::new(OpenAiCompatibleProvider::new(
@@ -137,6 +141,15 @@ mod tests {
         assert!(create_provider("ollama", None).is_ok());
     }
 
+    #[test]
+    fn factory_gemini() {
+        assert!(create_provider("gemini", Some("test-key")).is_ok());
+        assert!(create_provider("google", Some("test-key")).is_ok());
+        assert!(create_provider("google-gemini", Some("test-key")).is_ok());
+        // Should also work without key (will try CLI auth)
+        assert!(create_provider("gemini", None).is_ok());
+    }
+
     // ── OpenAI-compatible providers ──────────────────────────
 
     #[test]
@@ -301,6 +314,7 @@ mod tests {
             "anthropic",
             "openai",
             "ollama",
+            "gemini",
             "venice",
             "vercel",
             "cloudflare",