docs: Fix documentation accuracy and add missing Gemini provider

Corrections made:
- docs/architecture.md: Fix crate count from 9 to 8 (actual count)
- docs/tools.md: Fix code_search supported languages (kotlin -> haskell, scheme, racket)
- docs/CODE_SEARCH.md: Add missing Haskell and Scheme to supported languages list
- docs/providers.md: Add complete Gemini provider documentation section
- docs/configuration.md: Add Gemini configuration section

The Gemini provider (crates/g3-providers/src/gemini.rs) was fully implemented
but not documented. The code_search tool actually supports haskell and scheme
(via tree-sitter) but documentation incorrectly listed kotlin.

Agent: lamport

docs/configuration.md

@@ -89,6 +89,21 @@ use_oauth = true                 # Use OAuth (recommended)
 - **OAuth** (`use_oauth = true`): Opens browser for authentication, tokens refresh automatically
 - **Token** (`token = "..."`, `use_oauth = false`): Uses personal access token directly
 
+### Gemini Configuration
+
+```toml
+[providers.gemini.default]
+api_key = "your-google-api-key"  # Required: Your Google AI API key
+model = "gemini-2.0-flash"       # Model to use
+max_tokens = 8192
+temperature = 0.7
+```
+
+**Available Gemini models**:
+- `gemini-2.0-flash` (recommended)
+- `gemini-1.5-pro`
+- `gemini-1.5-flash`
+
 ### OpenAI Configuration
 
 ```toml