fix: block memory agent prose-skip responses at prompt and runtime levels

Observer prompt now explicitly requires XML observation blocks or empty
responses — prose explanations like "Skipping" are discarded. ResponseProcessor
logs a warning when non-XML content is received. Recording focus expanded to
include concrete debugging findings.

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

tests/sdk/prompts.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it } from 'bun:test';
+
+import { buildObservationPrompt } from '../../src/sdk/prompts.js';
+
+describe('buildObservationPrompt', () => {
+  it('instructs the observer to avoid prose skip responses', () => {
+    const prompt = buildObservationPrompt({
+      id: 1,
+      tool_name: 'exec_command',
+      tool_input: JSON.stringify({ cmd: 'pwd' }),
+      tool_output: JSON.stringify({ output: '/repo' }),
+      created_at_epoch: Date.now(),
+      cwd: '/repo',
+    });
+
+    expect(prompt).toContain('Return either one or more <observation>...</observation> blocks, or an empty response');
+    expect(prompt).toContain('Concrete debugging findings from logs, queue state, database rows, session routing, or code-path inspection');
+    expect(prompt).toContain('Never reply with prose such as "Skipping", "No substantive tool executions"');
+  });
+});