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.

docs/schemas/analysis.json

@@ -7,7 +7,9 @@
     },
     "requirements": {
       "type": "array",
-      "description": "concrete requirements / acceptance criteria, one per item"
+      "description": "concrete requirements / acceptance criteria, one per item",
+      "minItems": 1,
+      "items": { "type": "string" }
     },
     "affected_areas": {
       "type": "array",

docs/schemas/design.json

@@ -7,7 +7,9 @@
     },
     "components": {
       "type": "array",
-      "description": "components/files to add or change, one per item"
+      "description": "components/files to add or change, one per item",
+      "minItems": 1,
+      "items": { "type": "string" }
     },
     "risks": {
       "type": "array",

docs/schemas/impl_plan.json

@@ -3,7 +3,9 @@
   "properties": {
     "steps": {
       "type": "array",
-      "description": "ordered, individually verifiable implementation steps, one per item"
+      "description": "ordered, individually verifiable implementation steps, one per item",
+      "minItems": 1,
+      "items": { "type": "string" }
     },
     "verification": {
       "type": "array",