feat: initial llm-connect package scaffold

Copy markitect.llm module into standalone llm_connect package.
All markitect.* imports replaced with llm_connect.* equivalents.
LLMError base class inlined (no markitect.exceptions dependency).
Verified: from llm_connect import create_adapter works.

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

llm_connect/__init__.py

@@ -0,0 +1,67 @@
+"""
+llm-connect — Pluggable LLM adapters.
+
+Provides concrete :class:`LLMAdapter` implementations backed by
+OpenRouter (HTTP), Gemini, OpenAI, and Claude Code CLI (subprocess).
+
+Quick start::
+
+    from llm_connect import create_adapter
+
+    adapter = create_adapter("openrouter", model="anthropic/claude-sonnet-4")
+    response = adapter.execute_prompt(prompt, run_config)
+"""
+
+from llm_connect.models import RunConfig, LLMResponse
+from llm_connect.adapter import LLMAdapter, MockLLMAdapter, ErrorLLMAdapter
+from llm_connect.factory import create_adapter
+from llm_connect.openrouter import OpenRouterAdapter
+from llm_connect.claude_code import ClaudeCodeAdapter
+from llm_connect.gemini import GeminiAdapter
+from llm_connect.openai import OpenAIAdapter
+from llm_connect.config import LLMConfig, load_config
+from llm_connect.exceptions import (
+    LLMError,
+    LLMConfigurationError,
+    LLMAPIError,
+    LLMRateLimitError,
+    LLMTimeoutError,
+    LLMSubprocessError,
+)
+from llm_connect.embedding_adapter import EmbeddingAdapter
+from llm_connect.embedding_openai import OpenAICompatibleEmbeddingAdapter
+from llm_connect.embedding_cache import EmbeddingCache
+from llm_connect.embedding_factory import create_embedding_adapter
+from llm_connect.similarity import (
+    cosine_similarity,
+    similarity_matrix,
+    find_similar_pairs,
+)
+
+__all__ = [
+    "RunConfig",
+    "LLMResponse",
+    "LLMAdapter",
+    "MockLLMAdapter",
+    "ErrorLLMAdapter",
+    "create_adapter",
+    "OpenRouterAdapter",
+    "ClaudeCodeAdapter",
+    "GeminiAdapter",
+    "OpenAIAdapter",
+    "LLMConfig",
+    "load_config",
+    "LLMError",
+    "LLMConfigurationError",
+    "LLMAPIError",
+    "LLMRateLimitError",
+    "LLMTimeoutError",
+    "LLMSubprocessError",
+    "EmbeddingAdapter",
+    "OpenAICompatibleEmbeddingAdapter",
+    "EmbeddingCache",
+    "create_embedding_adapter",
+    "cosine_similarity",
+    "similarity_matrix",
+    "find_similar_pairs",
+]