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).

infrastructure/src/main/kotlin/com/correx/infrastructure/InfrastructureModule.kt

@@ -51,6 +51,8 @@ import com.correx.infrastructure.tools.DispatchingToolExecutor
 import com.correx.infrastructure.tools.SandboxedToolExecutor
 import com.correx.infrastructure.tools.ToolConfig
 import com.correx.infrastructure.tools.buildTools
+import com.correx.core.artifacts.kind.ArtifactKind
+import com.correx.core.artifacts.kind.DefaultArtifactKindRegistry
 import com.correx.infrastructure.workflow.FileSystemPromptLoader
 import com.correx.infrastructure.workflow.PromptLoader
 import com.correx.infrastructure.workflow.TomlWorkflowLoader
@@ -175,7 +177,11 @@ object InfrastructureModule {
             ),
         )
 
-    fun createWorkflowLoader(): WorkflowLoader = TomlWorkflowLoader()
+    fun createWorkflowLoader(extraKinds: List<ArtifactKind> = emptyList()): WorkflowLoader {
+        val registry = DefaultArtifactKindRegistry()
+        extraKinds.forEach { registry.register(it) }
+        return TomlWorkflowLoader(registry)
+    }
 
     fun createPromptLoader(): PromptLoader = FileSystemPromptLoader()