feat(llm): add embedding adapter with cache and similarity utils (S1.3)

Add OpenAI-compatible embedding support (works with both OpenAI and
OpenRouter), file-based embedding cache with content-digest invalidation,
and pure-Python cosine similarity utilities for downstream redundancy
detection.

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

markitect/llm/__init__.py

@@ -26,6 +26,15 @@ from markitect.llm.exceptions import (
     LLMTimeoutError,
     LLMSubprocessError,
 )
+from markitect.llm.embedding_adapter import EmbeddingAdapter
+from markitect.llm.embedding_openai import OpenAICompatibleEmbeddingAdapter
+from markitect.llm.embedding_cache import EmbeddingCache
+from markitect.llm.embedding_factory import create_embedding_adapter
+from markitect.llm.similarity import (
+    cosine_similarity,
+    similarity_matrix,
+    find_similar_pairs,
+)
 
 __all__ = [
     "create_adapter",
@@ -41,4 +50,11 @@ __all__ = [
     "LLMRateLimitError",
     "LLMTimeoutError",
     "LLMSubprocessError",
+    "EmbeddingAdapter",
+    "OpenAICompatibleEmbeddingAdapter",
+    "EmbeddingCache",
+    "create_embedding_adapter",
+    "cosine_similarity",
+    "similarity_matrix",
+    "find_similar_pairs",
 ]