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>

migrations/prompts/005_create_changes_and_debt.sql

@@ -0,0 +1,25 @@
+-- Phase 6: Incremental Execution tables
+-- artifact_changes: tracks detected changes to artifacts
+-- impact_debt: tracks suppressed recomputations
+
+CREATE TABLE IF NOT EXISTS artifact_changes (
+    id TEXT PRIMARY KEY,
+    artifact_id TEXT NOT NULL,
+    old_digest TEXT,
+    new_digest TEXT NOT NULL,
+    change_type TEXT NOT NULL,
+    detected_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+CREATE INDEX IF NOT EXISTS idx_changes_artifact ON artifact_changes(artifact_id);
+CREATE INDEX IF NOT EXISTS idx_changes_type ON artifact_changes(change_type);
+
+CREATE TABLE IF NOT EXISTS impact_debt (
+    id TEXT PRIMARY KEY,
+    artifact_id TEXT NOT NULL,
+    dependent_run_id TEXT NOT NULL,
+    change_magnitude REAL NOT NULL,
+    suppression_reason TEXT NOT NULL,
+    recorded_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+CREATE INDEX IF NOT EXISTS idx_debt_artifact ON impact_debt(artifact_id);
+CREATE INDEX IF NOT EXISTS idx_debt_run ON impact_debt(dependent_run_id);