chore(config): drop the unread retention.execution_history_days default

internal/config/README.md

@@ -212,9 +212,18 @@ purchaseDefaults := config.GetDefaultsByCategory("purchase_defaults")
 ### Data Retention
 
 - `retention.purchase_history_days`: `1095` (int) - 3 years
-- `retention.execution_history_days`: `90` (int)
 - `retention.recommendation_cache_hours`: `24` (int)
 
+Execution history retention is not a setting. It is the compile-time
+constant `DefaultExecutionTTLDays` (`constants.go`, 30 days), which
+`internal/server/handler.go` passes to `CleanupOldExecutions` on each
+cleanup run. That sweep deletes completed executions by `scheduled_date`
+and canceled ones by `updated_at` once they are older than the horizon,
+plus rows whose own `expires_at` lapsed that long ago (except statuses
+the health score counts). It never deletes failed executions. The plan health score's lookback window
+(`planHealthLookbackDays` in `internal/api/plan_health.go`) is the same
+constant, so changing one changes both.
+
 ### API Settings
 
 - `api.rate_limit_requests_per_minute`: `100` (int)

internal/config/defaults.go

@@ -285,14 +285,10 @@ var DefaultSettings = []ConfigSetting{
 		Description: "Days to retain purchase history (3 years default)",
 		UpdatedAt:   time.Time{},
 	},
-	{
-		Key:         "retention.execution_history_days",
-		Value:       90,
-		Type:        "int",
-		Category:    "retention",
-		Description: "Days to retain execution history records",
-		UpdatedAt:   time.Time{},
-	},
+	// Execution history retention is deliberately not a setting: it is
+	// DefaultExecutionTTLDays (constants.go), passed to CleanupOldExecutions
+	// by internal/server/handler.go and shared with the plan health
+	// lookback window (internal/api/plan_health.go). See issue #88.
 	{
 		Key:         "retention.recommendation_cache_hours",
 		Value:       24,

internal/config/defaults_test.go

@@ -218,7 +218,6 @@ func TestDefaultSettings_Retention(t *testing.T) {
 		expectedValue int
 	}{
 		{"retention.purchase_history_days", 1095},
-		{"retention.execution_history_days", 90},
 		{"retention.recommendation_cache_hours", 24},
 	}
 
@@ -233,6 +232,18 @@ func TestDefaultSettings_Retention(t *testing.T) {
 	}
 }
 
+// Execution retention is not a setting. It is the compile-time constant
+// DefaultExecutionTTLDays, which the cleanup handler passes to
+// CleanupOldExecutions and which the plan health lookback window reuses.
+// A "retention.execution_history_days" default (90, against a live value
+// of 30) once sat here with no reader, and it was cited as the retention
+// policy in review (issue #88). Keep it from coming back as an unwired key.
+func TestDefaultSettings_NoExecutionRetentionKey(t *testing.T) {
+	assert.Nil(t, GetDefaultSetting("retention.execution_history_days"),
+		"execution retention is DefaultExecutionTTLDays; a settings key for it would have no reader")
+	assert.Equal(t, 30, DefaultExecutionTTLDays)
+}
+
 func TestDefaultSettings_API(t *testing.T) {
 	tests := []struct {
 		key           string