refactor(artifact-graph): adopt zod v4 error message format

Update custom error messages from string format to zod v4 object format
using `{ error: 'message' }` convention.

Author: TabishB

src/core/artifact-graph/types.ts

@@ -2,19 +2,19 @@ import { z } from 'zod';
 
 // Artifact definition schema
 export const ArtifactSchema = z.object({
-  id: z.string().min(1, 'Artifact ID is required'),
-  generates: z.string().min(1, 'generates field is required'),
+  id: z.string().min(1, { error: 'Artifact ID is required' }),
+  generates: z.string().min(1, { error: 'generates field is required' }),
   description: z.string(),
-  template: z.string().min(1, 'template field is required'),
+  template: z.string().min(1, { error: 'template field is required' }),
   requires: z.array(z.string()).default([]),
 });
 
 // Full schema YAML structure
 export const SchemaYamlSchema = z.object({
-  name: z.string().min(1, 'Schema name is required'),
-  version: z.number().int().positive('Version must be a positive integer'),
+  name: z.string().min(1, { error: 'Schema name is required' }),
+  version: z.number().int().positive({ error: 'Version must be a positive integer' }),
   description: z.string().optional(),
-  artifacts: z.array(ArtifactSchema).min(1, 'At least one artifact required'),
+  artifacts: z.array(ArtifactSchema).min(1, { error: 'At least one artifact required' }),
 });
 
 // Derived TypeScript types