feat(llm): add Gemini adapter and process book-1-chapter-05

Add GeminiAdapter calling Google's Generative Language REST API
(default model: gemini-2.5-flash). Register "gemini" as third
provider in the factory and CLI. Add rate-limit retry with
exponential backoff to the pipeline's _call_llm helper. Increase
default max_tokens from 2000 to 4096.

Process book-1-chapter-05 via Gemini free tier — 1 new entity
extracted (necessaries-conveniencies-and-amusements-of-life),
41 existing entities correctly skipped by dedup. Canonical set
now at 42 unique entities.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

markitect/llm/factory.py

@@ -11,6 +11,7 @@ from markitect.llm.exceptions import LLMConfigurationError
 _PROVIDERS: Dict[str, str] = {
     "openrouter": "markitect.llm.openrouter.OpenRouterAdapter",
     "claude-code": "markitect.llm.claude_code.ClaudeCodeAdapter",
+    "gemini": "markitect.llm.gemini.GeminiAdapter",
 }
 
 
@@ -24,10 +25,10 @@ def create_adapter(
     """Instantiate an :class:`LLMAdapter` for the given *provider*.
 
     Args:
-        provider: ``"openrouter"`` or ``"claude-code"``.
+        provider: ``"openrouter"``, ``"claude-code"``, or ``"gemini"``.
         model: Model name (passed to the adapter constructor).
-        api_key: Explicit API key (OpenRouter only).
-        system_prompt: Optional system prompt (OpenRouter only).
+        api_key: Explicit API key (OpenRouter / Gemini).
+        system_prompt: Optional system prompt (OpenRouter / Gemini).
         **kwargs: Extra keyword arguments forwarded to the adapter.
 
     Returns:
@@ -50,7 +51,7 @@ def create_adapter(
     mod = importlib.import_module(module_path)
     cls = getattr(mod, class_name)
 
-    if provider == "openrouter":
+    if provider in ("openrouter", "gemini"):
         return cls(model=model, api_key=api_key, system_prompt=system_prompt, **kwargs)
     elif provider == "claude-code":
         return cls(model=model, **kwargs)