fix: never clean up observability artifacts — partial file persists after finalize

Removing the _partial-e2e.json deletion from finalize(). These are small files
on a local disk and their persistence is the whole point of observability.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/helpers/eval-store.ts

@@ -444,9 +444,6 @@ export class EvalCollector {
       tests: this.tests,
     };
 
-    // Delete partial file now that we're writing the final
-    try { fs.unlinkSync(path.join(this.evalDir, '_partial-e2e.json')); } catch { /* may not exist */ }
-
     // Write eval file
     fs.mkdirSync(this.evalDir, { recursive: true });
     const dateStr = timestamp.replace(/[:.]/g, '').replace('T', '-').slice(0, 15);

test/helpers/observability.test.ts

@@ -153,7 +153,7 @@ describe('eval-store observability', () => {
     expect(partial.tests[1].last_tool_call).toBe('Bash(ls)');
   });
 
-  test('7: finalize() deletes partial file', async () => {
+  test('7: finalize() preserves partial file alongside final', async () => {
     const evalDir = path.join(tmpDir, 'evals');
     const collector = new EvalCollector('e2e', evalDir);
 
@@ -167,9 +167,10 @@ describe('eval-store observability', () => {
 
     await collector.finalize();
 
-    expect(fs.existsSync(partialPath)).toBe(false);
+    // Partial file preserved for observability — never cleaned up
+    expect(fs.existsSync(partialPath)).toBe(true);
 
-    // Final eval file should exist
+    // Final eval file should also exist
     const files = fs.readdirSync(evalDir).filter(f => f.endsWith('.json') && !f.startsWith('_'));
     expect(files.length).toBeGreaterThanOrEqual(1);
   });