feat: WP-0001 foundation + WP-0002 core extensions

WP-0001 — Foundation & GAAF Baseline
- SCOPE.md, ARCHITECTURE-LAYERS.md, contracts/ tree
- .claude/rules/ stubs filled (architecture, stack, boundary)
- 57 tests (pytest), pyproject.toml with ruff+mypy, CI workflow

WP-0002 — Core Extensions (FR-4 + FR-3)
- FR-4: BudgetTracker (thread-safe) + LLMBudgetExceededError +
  optional RunConfig.budget_tracker + enforcement in all adapters
- FR-3: async_execute_prompt on LLMAdapter ABC (asyncio.to_thread
  fallback) + native asyncio.create_subprocess_exec in ClaudeCodeAdapter

81 tests passing.

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

llm_connect/__init__.py

@@ -12,7 +12,7 @@ Quick start::
     response = adapter.execute_prompt(prompt, run_config)
 """
 
-from llm_connect.models import RunConfig, LLMResponse
+from llm_connect.models import RunConfig, LLMResponse, BudgetTracker
 from llm_connect.adapter import LLMAdapter, MockLLMAdapter, ErrorLLMAdapter
 from llm_connect.factory import create_adapter
 from llm_connect.openrouter import OpenRouterAdapter
@@ -27,6 +27,7 @@ from llm_connect.exceptions import (
     LLMRateLimitError,
     LLMTimeoutError,
     LLMSubprocessError,
+    LLMBudgetExceededError,
 )
 from llm_connect.embedding_adapter import EmbeddingAdapter
 from llm_connect.embedding_openai import OpenAICompatibleEmbeddingAdapter
@@ -41,6 +42,7 @@ from llm_connect.similarity import (
 __all__ = [
     "RunConfig",
     "LLMResponse",
+    "BudgetTracker",
     "LLMAdapter",
     "MockLLMAdapter",
     "ErrorLLMAdapter",
@@ -57,6 +59,7 @@ __all__ = [
     "LLMRateLimitError",
     "LLMTimeoutError",
     "LLMSubprocessError",
+    "LLMBudgetExceededError",
     "EmbeddingAdapter",
     "OpenAICompatibleEmbeddingAdapter",
     "EmbeddingCache",