revert: remove overengineered summary salvage logic (#1718) (#1850)

The synthetic summary salvage feature created fake summaries from observation
data when the AI returned <observation> instead of <summary> tags. This was
overengineered — missing a summary is preferable to fabricating one from
observation fields that don't map cleanly to summary semantics.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/worker/agents/response-processor.test.ts

@@ -319,9 +319,7 @@ describe('ResponseProcessor', () => {
       );
 
       const [, , , summary] = mockStoreObservations.mock.calls[0];
-      // #1718: When observations exist without <summary> tags, a synthetic summary is salvaged
-      expect(summary).not.toBeNull();
-      expect(summary.notes).toContain('Salvaged from');
+      expect(summary).toBeNull();
     });
   });