feat(prompts): implement Phase 6 - Incremental Execution (FR-7, FR-8)

Add change detection, structural diff-based impact analysis,
configurable-depth incremental recomputation with circular suppression,
and impact debt tracking.

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

markitect/prompts/incremental/__init__.py

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+"""
+Incremental execution for prompt artifacts.
+
+Implements FR-7: Incremental Recomputation
+Implements FR-8: Impact Analysis
+
+- FR-7.1: Change detection with persistent records
+- FR-7.2: Configurable-depth incremental recomputation
+- FR-8.1: Structural diff-based impact analysis
+- FR-8.2: Impact threshold decisions
+"""
+
+from markitect.prompts.incremental.models import (
+    ChangeType,
+    ArtifactChange,
+    ImpactDebt,
+    RecomputeConfig,
+    RecomputeResult,
+)
+from markitect.prompts.incremental.detector import ChangeDetector
+from markitect.prompts.incremental.metrics import (
+    structural_diff_ratio,
+    line_diff_ratio,
+    calculate_change_magnitude,
+)
+from markitect.prompts.incremental.impact import ImpactAnalyzer
+from markitect.prompts.incremental.engine import IncrementalExecutionEngine
+
+__all__ = [
+    # Models
+    "ChangeType",
+    "ArtifactChange",
+    "ImpactDebt",
+    "RecomputeConfig",
+    "RecomputeResult",
+    # Detector
+    "ChangeDetector",
+    # Metrics
+    "structural_diff_ratio",
+    "line_diff_ratio",
+    "calculate_change_magnitude",
+    # Impact
+    "ImpactAnalyzer",
+    # Engine
+    "IncrementalExecutionEngine",
+]