feat(prompts): implement Phase 5 - Dependency Tracking (FR-6)

Add directed dependency graph with cycle detection, topological sort,
and query service for finding dependents/dependencies transitively.

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

markitect/prompts/dependencies/__init__.py

@@ -0,0 +1,40 @@
+"""
+Dependency tracking for prompt artifacts.
+
+Implements FR-6: Dependency Tracking
+- FR-6.1: Directed dependency edges between artifacts
+- FR-6.2: Cross-space dependency graph
+- FR-6.3: Circular dependency detection
+"""
+
+from markitect.prompts.dependencies.models import (
+    EdgeType,
+    DependencyEdge,
+    DependencyGraph,
+    CircularDependencyError,
+)
+from markitect.prompts.dependencies.repository import (
+    IDependencyRepository,
+    SQLiteDependencyRepository,
+    DependencyRepositoryError,
+    DuplicateDependencyError,
+)
+from markitect.prompts.dependencies.graph import GraphBuilder
+from markitect.prompts.dependencies.queries import DependencyQueryService
+
+__all__ = [
+    # Models
+    "EdgeType",
+    "DependencyEdge",
+    "DependencyGraph",
+    "CircularDependencyError",
+    # Repository
+    "IDependencyRepository",
+    "SQLiteDependencyRepository",
+    "DependencyRepositoryError",
+    "DuplicateDependencyError",
+    # Graph
+    "GraphBuilder",
+    # Queries
+    "DependencyQueryService",
+]