P4: budget battery — DBBudget (rolling 7-day) over run.Budget

Second Tier-2 battery, plugging into run.Ports.Budget:
- budget.go: skillexec's BudgetTracker / NoOpBudget / DBBudget moved clean
  (stdlib only). Check/Commit match run.Budget exactly (compile-time proof in
  run.go: NoOpBudget and *DBBudget are run.Budget).
- storage.go: the BudgetStorage seam + SkillBudget domain, split out of mort's
  GORM file (the GORM impl stays in mort).
- memory.go: NewMemory() — zero-dependency in-process BudgetStorage with the
  7-day rolling-window rollover in Add.

Tests: per-user cap enforced, window rolls over after 7 days, NoOp always
allows. CI invariant: core imports ZERO from the budget battery.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

budget/budget_test.go

@@ -0,0 +1,44 @@
+package budget
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestDBBudgetRollingWindow(t *testing.T) {
+	ctx := context.Background()
+	mem := NewMemory()
+	now := time.Now()
+	clock := func() time.Time { return now }
+	b := NewDBBudget(mem, func() float64 { return 100 }, nil, clock)
+
+	// Under cap: allowed.
+	if err := b.Check(ctx, "u"); err != nil {
+		t.Fatalf("fresh caller should pass: %v", err)
+	}
+	b.Commit(ctx, "u", 60)
+	if err := b.Check(ctx, "u"); err != nil {
+		t.Fatalf("60/100 should pass: %v", err)
+	}
+	// Over cap: rejected.
+	b.Commit(ctx, "u", 50) // 110 total
+	if err := b.Check(ctx, "u"); !errors.Is(err, ErrBudgetExceeded) {
+		t.Fatalf("110/100 should be ErrBudgetExceeded, got %v", err)
+	}
+	// Window rolls over after 7 days: allowed again.
+	now = now.Add(8 * 24 * time.Hour)
+	b.Commit(ctx, "u", 1) // triggers rollover inside Add
+	if err := b.Check(ctx, "u"); err != nil {
+		t.Fatalf("after window rollover should pass: %v", err)
+	}
+}
+
+func TestNoOpBudgetAlwaysAllows(t *testing.T) {
+	b := NewNoOpBudget()
+	if err := b.Check(context.Background(), "anyone"); err != nil {
+		t.Fatalf("NoOp must always allow: %v", err)
+	}
+	b.Commit(context.Background(), "anyone", 1e9) // no-op
+}