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/__init__.py

@@ -15,6 +15,7 @@ Quick start::
 from markitect.llm.factory import create_adapter
 from markitect.llm.openrouter import OpenRouterAdapter
 from markitect.llm.claude_code import ClaudeCodeAdapter
+from markitect.llm.gemini import GeminiAdapter
 from markitect.llm.config import LLMConfig, load_config
 from markitect.llm.exceptions import (
     LLMError,
@@ -29,6 +30,7 @@ __all__ = [
     "create_adapter",
     "OpenRouterAdapter",
     "ClaudeCodeAdapter",
+    "GeminiAdapter",
     "LLMConfig",
     "load_config",
     "LLMError",