fix(screenshots): #551 landing fixes

Pin the deprecation warning on every /api/screenshots route. The PR's test
sends two GET /api/screenshots requests and checks that exactly one warning
comes out, which proves the once-flag but not the individual calls: the
POST and GET /:name warn calls could be deleted and it would stay green. A
new it.each sends one request per route (GET list, a non-multipart POST that
reaches the handler before the content-type check, and GET /:name for a
missing file) on the fresh per-test harness, and each case asserts exactly
one warning naming POST /api/sessions/:id/paste-image. Removing any single
warn call now fails its own case (checked by deleting each call in turn).

The deprecation's CHANGELOG note, required by docs/versioning-policy.md for a
deprecated covered surface, rides the consolidated release changeset rather
than a file here, since the PR added none.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/routes/system-routes.test.ts

@@ -696,6 +696,31 @@ describe('system-routes', () => {
         warn.mockRestore();
       }
     });
+
+    // One request per route on the fresh per-test harness, so each warn call is pinned on its own
+    // (the once-flag would let the first route's warning mask a missing call on the others).
+    it.each([
+      { label: 'GET /api/screenshots', method: 'GET' as const, url: '/api/screenshots' },
+      {
+        label: 'POST /api/screenshots',
+        method: 'POST' as const,
+        url: '/api/screenshots',
+        payload: { file: 'data' },
+        headers: { 'content-type': 'application/json' },
+      },
+      { label: 'GET /api/screenshots/:name', method: 'GET' as const, url: '/api/screenshots/nonexistent.png' },
+    ])('$label warns that it is deprecated', async ({ method, url, payload, headers }) => {
+      const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+      try {
+        mockedExistsSync.mockReturnValue(false);
+        await harness.app.inject({ method, url, payload, headers });
+        const deprecations = warn.mock.calls.filter((c) => String(c[0]).includes('/api/screenshots'));
+        expect(deprecations).toHaveLength(1);
+        expect(String(deprecations[0][0])).toContain('POST /api/sessions/:id/paste-image');
+      } finally {
+        warn.mockRestore();
+      }
+    });
   });
 
   // ========== GET /api/screenshots/:name ==========