fix(research): flatten source_dossier schema to correx's flat JsonSchema

The schema declared sources as an array of objects, which correx's flat
JsonSchema model can't represent (JsonSchemaProperty has no nested
properties). The strict config loader rejected it at startup, silently
disabling the source_dossier artifact kind. Flatten sources to an array
of strings (URL-prefixed per-source notes), update the gather prompt to
match, and add ResearchSchemaLoadTest to guard every shipped research
schema against the strict loader.

core/artifacts/src/test/kotlin/com/correx/core/artifacts/kind/ResearchSchemaLoadTest.kt

@@ -0,0 +1,47 @@
+package com.correx.core.artifacts.kind
+
+import kotlinx.serialization.json.Json
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.Test
+import java.nio.file.Path
+import kotlin.io.path.exists
+import kotlin.io.path.readText
+
+/**
+ * The research artifact schemas (docs/schemas/research_*.json, source_dossier.json) must parse with
+ * the same strict deserializer the server uses to load config artifact kinds. correx's JsonSchema is
+ * flat — array properties take only scalar `items` — so a schema with array-of-object items fails to
+ * load at startup (silently disabling the kind). This guards every shipped research schema.
+ */
+class ResearchSchemaLoadTest {
+
+    private fun repoFile(relative: String): Path {
+        var dir: Path? = Path.of("").toAbsolutePath()
+        while (dir != null) {
+            val candidate = dir.resolve(relative)
+            if (candidate.exists()) return candidate
+            dir = dir.parent
+        }
+        error("could not locate $relative")
+    }
+
+    private fun load(name: String): JsonSchema =
+        Json.decodeFromString(JsonSchema.serializer(), repoFile("docs/schemas/$name").readText())
+
+    @Test
+    fun `research schemas parse with the strict loader`() {
+        for (name in listOf("research_plan.json", "source_dossier.json", "research_report.json")) {
+            val schema = load(name)
+            assertEquals("object", schema.type, "$name root must be an object")
+        }
+    }
+
+    @Test
+    fun `source dossier sources is an array of strings`() {
+        val sources = load("source_dossier.json").properties.getValue("sources")
+        assertEquals("array", sources.type)
+        assertEquals("string", sources.items?.type, "items must be scalar — the flat JsonSchema model")
+        assertTrue(sources.minItems >= 1)
+    }
+}