feat(artifacts): config-driven custom artifact kinds with schema validation

Users declare artifact kinds in [[artifacts]] (id + schema_path to a JSON-schema file + llm_emitted); ConfigArtifactKind registers them at startup via createWorkflowLoader(extraKinds). New JsonSchemaValidator validates any non-built-in kind generically against its declared deriveJsonSchema(), so an LLM-emitted custom kind is checked against its shape, never trusted. Removed the dead payloadSerializer from the ArtifactKind contract. Schema source is path-to-file (not inline TOML — the hand-rolled parser can't nest).

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

@@ -1,10 +1,7 @@
 package com.correx.core.artifacts.kind
 
-import kotlinx.serialization.KSerializer
-
 interface ArtifactKind {
     val id: String
-    val payloadSerializer: KSerializer<*>
     fun deriveJsonSchema(): JsonSchema
     val llmEmitted: Boolean
 }

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

@@ -0,0 +1,16 @@
+package com.correx.core.artifacts.kind
+
+/**
+ * A user-declared artifact kind whose schema comes from configuration rather than
+ * a compile-time Kotlin type. The declared [schema] is authoritative: content is
+ * validated against it generically (see the artifact-payload validator), so an
+ * [llmEmitted] config kind is checked against its declared shape, never a
+ * hardcoded type check.
+ */
+data class ConfigArtifactKind(
+    override val id: String,
+    private val schema: JsonSchema,
+    override val llmEmitted: Boolean = false,
+) : ArtifactKind {
+    override fun deriveJsonSchema(): JsonSchema = schema
+}

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

@@ -1,10 +1,7 @@
 package com.correx.core.artifacts.kind
 
-import kotlinx.serialization.KSerializer
-
 object FileWrittenKind : ArtifactKind {
     override val id: String = "file_written"
-    override val payloadSerializer: KSerializer<*> = FileWrittenPayload.serializer()
     override val llmEmitted: Boolean = false
 
     override fun deriveJsonSchema(): JsonSchema = JsonSchema(

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

@@ -1,10 +1,7 @@
 package com.correx.core.artifacts.kind
 
-import kotlinx.serialization.KSerializer
-
 object ProcessResultKind : ArtifactKind {
     override val id: String = "process_result"
-    override val payloadSerializer: KSerializer<*> = ProcessResultArtifact.serializer()
     override val llmEmitted: Boolean = false
 
     override fun deriveJsonSchema(): JsonSchema = JsonSchema(

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

@@ -29,6 +29,21 @@ class ArtifactKindRegistryTest {
         assertEquals(false, registry.get("process_result")!!.llmEmitted)
     }
 
+    @Test
+    fun `config-declared kind registers and exposes its declared schema`() {
+        val schema = JsonSchema(
+            type = "object",
+            properties = mapOf("x" to JsonSchemaProperty(type = "string")),
+            required = listOf("x"),
+        )
+        registry.register(ConfigArtifactKind(id = "custom", schema = schema, llmEmitted = true))
+
+        val kind = registry.get("custom")
+        assertNotNull(kind)
+        assertEquals(true, kind!!.llmEmitted)
+        assertEquals(schema, kind.deriveJsonSchema())
+    }
+
     @Test
     fun `FileWrittenKind schema has required fields`() {
         val schema = FileWrittenKind.deriveJsonSchema()