feat(llm): extract adapter layer for standalone llm-connect package (S1+S2)

Stage 1 — Decouple:
- Move RunConfig + LLMResponse to markitect/llm/models.py (canonical)
- Move LLMAdapter + Mock/ErrorLLMAdapter to markitect/llm/adapter.py
- markitect/prompts/execution/models.py and llm_adapter.py become re-export shims
- All 4 adapters + factory.py updated to import from markitect.llm.*
- Parameterize app_name in toml_config.py (resolve_llm, get_default_layers,
  get_preference_layers): paths and env var now derived from app_name arg
- Add tests/test_llm_isolation.py: 7 isolation + backward-compat tests

Stage 2 — Extract:
- Standalone llm-connect package created at ~/llm-connect/
- All 18 llm files copied; markitect.* imports replaced with llm_connect.*
- LLMError base inlined in llm_connect/exceptions.py (no markitect dep)
- llm-connect installed into markitect-venv; declared in pyproject.toml

Smoke test: markitect llm-check succeeds (live Gemini API call).
Backward compat: markitect.prompts.execution.{models,llm_adapter} still work.

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

markitect/llm/__init__.py

@@ -12,6 +12,8 @@ Quick start::
     response = adapter.execute_prompt(prompt, run_config)
 """
 
+from markitect.llm.models import RunConfig, LLMResponse
+from markitect.llm.adapter import LLMAdapter, MockLLMAdapter, ErrorLLMAdapter
 from markitect.llm.factory import create_adapter
 from markitect.llm.openrouter import OpenRouterAdapter
 from markitect.llm.claude_code import ClaudeCodeAdapter
@@ -37,6 +39,11 @@ from markitect.llm.similarity import (
 )
 
 __all__ = [
+    "RunConfig",
+    "LLMResponse",
+    "LLMAdapter",
+    "MockLLMAdapter",
+    "ErrorLLMAdapter",
     "create_adapter",
     "OpenRouterAdapter",
     "ClaudeCodeAdapter",