gate: enforce destructive budget for script step targets

[davidahmann]

Problem

PR #62 review feedback identified a policy gap: destructive-budget enforcement only checked top-level intent.targets, which allowed scripted destructive targets in intent.script.steps[*].targets to avoid budget accounting.

Changes

• Route destructive-budget eligibility through a new helper that inspects script step targets first and falls back to top-level targets when script targets are absent.
• Keep behavior unchanged for non-script intents and script intents without per-step targets.
• Add unit coverage for helper behavior:
  • script targets are detected
  • fallback to top-level targets still works
• Add CLI regression coverage to confirm repeated scripted destructive calls trip destructive_budget_exceeded.

Validation

• go test ./cmd/gait -run 'GateIntent|DestructiveBudgetAppliesToScriptTargets' -count=1
• make prepush-full
• pre-push hook make prepush on git push