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>

migrations/prompts/004_create_dependencies.sql

@@ -0,0 +1,36 @@
+-- Migration 004: Create prompt dependencies table
+-- Implements FR-6: Dependency Tracking
+-- FR-6.1: Directed dependency edges between artifacts
+-- FR-6.2: Cross-space dependency graph
+-- Date: 2026-02-08
+
+-- Prompt dependency edges table
+CREATE TABLE IF NOT EXISTS prompt_dependencies (
+    id TEXT PRIMARY KEY,
+    source_artifact_id TEXT NOT NULL,
+    target_artifact_id TEXT NOT NULL,
+    run_id TEXT NOT NULL,
+    edge_type TEXT NOT NULL,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    UNIQUE(source_artifact_id, target_artifact_id, run_id)
+);
+
+-- Note: Foreign keys documented but not enforced (matches existing pattern)
+-- source_artifact_id REFERENCES prompt_artifacts(id)
+-- target_artifact_id REFERENCES prompt_artifacts(id)
+-- run_id REFERENCES prompt_runs(id)
+
+-- Indexes for performance
+CREATE INDEX IF NOT EXISTS idx_deps_source ON prompt_dependencies(source_artifact_id);
+CREATE INDEX IF NOT EXISTS idx_deps_target ON prompt_dependencies(target_artifact_id);
+CREATE INDEX IF NOT EXISTS idx_deps_run ON prompt_dependencies(run_id);
+CREATE INDEX IF NOT EXISTS idx_deps_type ON prompt_dependencies(edge_type);
+CREATE INDEX IF NOT EXISTS idx_deps_source_target ON prompt_dependencies(source_artifact_id, target_artifact_id);
+
+-- Column documentation:
+-- id: Unique UUID for this dependency edge
+-- source_artifact_id: Artifact that depends on the target
+-- target_artifact_id: Artifact that is depended upon
+-- run_id: The execution run that established this dependency
+-- edge_type: requires, generates, includes
+-- created_at: When this dependency was recorded