Aggregate highest-risk decision across plans in runAttackSuite

[xhulz]

src/core/attackSuite.ts — when multiple attack plans run, each has its own assessment.decision.
There is no summary field that reflects the worst-case decision across all plans.

Example: if one plan is fix-now and another is no-action, the suite result has no
top-level field signaling that action is needed.

Add a suiteDecision field to AttackSuiteResult that is the highest-severity decision
across all planResults (fix-now > investigate > review > no-action).