Chat autosend, week rollover and ai_runs purge schedules (Feature 20)

shopping_chat_autosend fires at chat_autosend_dow/time for the coming
week and posts its Supply run through SendToChat when a webhook is
stored, list_ready's Chat channel is on and the list exists unsent;
otherwise it logs why (key list_ready:chat:{weekStart}). week_rollover
freezes the previous week at Monday 00:05 through Freeze, judging the
week ended by the fire time. ai_runs_purge calls harness.Purge daily at
03:00 with the fire time less RunRetention. Fake-clock tests cover each.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/ai/harness/schedule.go

@@ -0,0 +1,46 @@
+package harness
+
+import (
+	"context"
+	"errors"
+	"log/slog"
+	"time"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/household"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/scheduler"
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/store"
+)
+
+// PurgeTime is when, every day in the household zone, ai_runs_purge runs.
+// It is fixed, not a setting.
+const PurgeTime = "03:00"
+
+// PurgeSchedule is the ai_runs_purge schedule over st: every day at
+// PurgeTime it deletes the ai_runs rows created more than RunRetention
+// before the fire time with Purge, its period scheduler.Daily keyed
+// ai_runs_purge:YYYY-MM-DD. A purge caught up late deletes no row younger
+// than RunRetention; one that fails is the period's error and is not
+// retried, and the next day's purge deletes what it left.
+func PurgeSchedule(st *store.Store, logger *slog.Logger) (scheduler.Schedule, error) {
+	if st == nil {
+		return scheduler.Schedule{}, errors.New("harness: purge schedule: nil store")
+	}
+	if logger == nil {
+		return scheduler.Schedule{}, errors.New("harness: purge schedule: nil logger")
+	}
+	return scheduler.Schedule{
+		Name: scheduler.AIRunsPurge,
+		Rule: func(household.Settings) scheduler.Rule {
+			return scheduler.Daily(PurgeTime)
+		},
+		Run: func(ctx context.Context, p scheduler.Period) error {
+			cutoff := p.FireAt.Add(-RunRetention)
+			n, err := Purge(ctx, st, cutoff)
+			if err != nil {
+				return err
+			}
+			logger.InfoContext(ctx, "ai_runs purged", "period", p.Key, "deleted", n, "created_before", cutoff.UTC().Format(time.RFC3339))
+			return nil
+		},
+	}, nil
+}