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/run.go

@@ -0,0 +1,9 @@
+package budget
+
+import "gitea.stevedudenhoeffer.com/steve/executus/run"
+
+// The budget trackers plug directly into run.Ports.Budget (Check/Commit match).
+var (
+	_ run.Budget = NoOpBudget{}
+	_ run.Budget = (*DBBudget)(nil)
+)