refactor: centralize typed GSDToolsError construction

sdk/src/gsd-tools-error.test.ts

@@ -0,0 +1,16 @@
+import { describe, expect, it } from 'vitest';
+import { GSDToolsError } from './gsd-tools-error.js';
+
+describe('GSDToolsError constructors', () => {
+  it('builds timeout-classified errors', () => {
+    const err = GSDToolsError.timeout('timeout', 'state', ['load'], '', 1000);
+    expect(err.classification).toEqual({ kind: 'timeout', timeoutMs: 1000 });
+    expect(err.exitCode).toBeNull();
+  });
+
+  it('builds failure-classified errors', () => {
+    const err = GSDToolsError.failure('boom', 'state', ['load'], 1);
+    expect(err.classification).toEqual({ kind: 'failure' });
+    expect(err.exitCode).toBe(1);
+  });
+});