fix(benchmarks): Reject removed sequence config key

Reject the old sequence suite config key with an explicit migration
message so migrated benchmarks do not silently drop sequence checks.

Co-Authored-By: OpenAI Codex <noreply@openai.com>

Author: cameroncooke

src/benchmarks/claude-ui/__tests__/claude-ui-benchmark.test.ts

@@ -315,6 +315,19 @@ describe('Claude UI benchmark analysis', () => {
         'weather.yml',
       ),
     ).toThrow('weather.yml.allowedVariance: removed; baselines are observed data only');
+
+    expect(() =>
+      readConfig(
+        {
+          name: 'weather',
+          prompt: 'prompt.md',
+          sequence: { mode: 'fail' },
+        },
+        'weather.yml',
+      ),
+    ).toThrow(
+      'weather.yml.sequence: removed; use baselineToolSequence for observed sequence reporting',
+    );
   });
 
   it('rejects malformed failure pattern regexes when loading config', () => {

src/benchmarks/claude-ui/config.ts

@@ -247,6 +247,7 @@ function rejectRemovedConfigKeys(raw: Record<string, unknown>, source: string):
     allowedVariance: 'removed; baselines are observed data only',
     expectedFailures: 'removed; benchmark stumbles are observed data',
     expectedToolSequence: 'renamed to baselineToolSequence',
+    sequence: 'removed; use baselineToolSequence for observed sequence reporting',
   };
   for (const [key, message] of Object.entries(removedKeys)) {
     if (raw[key] !== undefined) throw new Error(`${source}.${key}: ${message}`);