feat(validation): enforce array minItems/items in artifact schemas (role-reliability §3)

The JSON-schema artifact validator enforced `required` keys but not array
contents, so a model could emit `"requirements": []` (or `"steps": []`,
`"components": []`) and pass — a misread brief sailing through as a valid
artifact. That empty-list gap is the hole under role-reliability §3's
keystone ("force the analyst to define done; verifiability propagates
downstream").

- JsonSchemaProperty gains `minItems` and `items` (recursive), letting a
  schema declare a required-non-empty / element-typed array.
- JsonSchemaValidator checks array length >= minItems and each element's
  declared item type (with index in the message). Pure + deterministic;
  single-return to stay within the detekt ReturnCount budget.
- Role-pipeline schemas tightened: analysis.requirements, design.components,
  impl_plan.steps → minItems 1 + items string. Prompt-free — these arrays
  were already `required`; this only forbids them being empty.

§4 (force a non-empty `alternativesConsidered` on the architect) is
deliberately NOT taken: it contradicts the standing "don't raise
alternatives unnecessarily" preference the architect prompt already encodes.

core/artifacts/src/main/kotlin/com/correx/core/artifacts/kind/JsonSchema.kt

@@ -15,4 +15,10 @@ data class JsonSchemaProperty(
     val type: String,
     val description: String = "",
     val default: String? = null,
+    // For `type == "array"`: minimum element count (0 = unconstrained) and the element
+    // schema. Lets a schema declare a required-non-empty list — the deterministic guard
+    // behind the role pipeline's "define done" contract (an empty `requirements`/`steps`
+    // array is a misread brief, not a valid artifact).
+    val minItems: Int = 0,
+    val items: JsonSchemaProperty? = null,
 )