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/models.py

@@ -5,8 +5,53 @@ These classes are the canonical definitions; they are re-exported by
 markitect.prompts.execution.models for backward compatibility.
 """
 
+import threading
 from dataclasses import dataclass, field
-from typing import Dict, Any
+from typing import Dict, Any, Optional
+
+
+class BudgetTracker:
+    """Shared token budget for a call or delegation chain.
+
+    Thread-safe. Tracks cumulative token spend across multiple adapter
+    calls. Raises ``LLMBudgetExceededError`` when the cap is exceeded.
+
+    Example::
+
+        tracker = BudgetTracker(total=4000)
+        config = RunConfig(budget_tracker=tracker)
+        # All adapter calls sharing this config will consume from the same cap.
+    """
+
+    def __init__(self, total: int) -> None:
+        if total <= 0:
+            raise ValueError(f"BudgetTracker total must be positive, got {total}")
+        self.total = total
+        self.spent = 0
+        self._lock = threading.Lock()
+
+    def remaining(self) -> int:
+        """Return tokens remaining in the budget."""
+        return max(0, self.total - self.spent)
+
+    def consume(self, tokens: int) -> None:
+        """Record *tokens* as spent. Raises ``LLMBudgetExceededError`` if cap exceeded."""
+        from llm_connect.exceptions import LLMBudgetExceededError  # avoid circular at module load
+
+        with self._lock:
+            new_spent = self.spent + tokens
+            if new_spent > self.total:
+                raise LLMBudgetExceededError(
+                    f"Token budget exceeded: {new_spent} tokens used, cap is {self.total}",
+                    total=self.total,
+                    spent=self.spent,
+                    requested=tokens,
+                    context={"total": self.total, "spent": self.spent, "requested": tokens},
+                )
+            self.spent = new_spent
+
+    def __repr__(self) -> str:
+        return f"BudgetTracker(total={self.total}, spent={self.spent}, remaining={self.remaining()})"
 
 
 @dataclass
@@ -30,9 +75,10 @@ class RunConfig:
     max_depth: int = 3
     skip_if_exists: bool = True
     timeout_seconds: int = 300
+    budget_tracker: Optional["BudgetTracker"] = field(default=None, repr=False)
 
     def to_dict(self) -> Dict[str, Any]:
-        """Convert to dictionary."""
+        """Convert to dictionary. ``budget_tracker`` is excluded (runtime object)."""
         return {
             "model_name": self.model_name,
             "temperature": self.temperature,