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/memory.go

@@ -0,0 +1,56 @@
+package budget
+
+import (
+	"context"
+	"sync"
+	"time"
+)
+
+// Memory is a zero-dependency in-process BudgetStorage: per-user rolling-window
+// usage held in memory (lost on restart). The default behind DBBudget for a
+// light host or tests; mort uses its GORM Storage, contrib/store adds SQLite.
+type Memory struct {
+	mu   sync.Mutex
+	rows map[string]*SkillBudget
+}
+
+// NewMemory returns an empty in-memory BudgetStorage.
+func NewMemory() *Memory { return &Memory{rows: map[string]*SkillBudget{}} }
+
+var _ BudgetStorage = (*Memory)(nil)
+
+func (m *Memory) Initialize(context.Context) error { return nil }
+
+func (m *Memory) Get(_ context.Context, userID string) (*SkillBudget, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	r, ok := m.rows[userID]
+	if !ok {
+		return nil, nil
+	}
+	cp := *r // copy out so callers can't mutate our row
+	return &cp, nil
+}
+
+func (m *Memory) Add(_ context.Context, userID string, secondsUsed float64, now time.Time) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	r, ok := m.rows[userID]
+	if !ok {
+		m.rows[userID] = &SkillBudget{
+			UserID: userID, WindowStart: now,
+			SecondsUsed: secondsUsed, RunsCount: 1, UpdatedAt: now,
+		}
+		return nil
+	}
+	// Roll the window over if it's older than the window length.
+	if now.Sub(r.WindowStart) >= budgetWindow {
+		r.WindowStart = now
+		r.SecondsUsed = 0
+		r.RunsCount = 0
+	}
+	r.SecondsUsed += secondsUsed
+	r.RunsCount++
+	r.UpdatedAt = now
+	return nil
+}