feat: feature-flag Codex SDK runner (#218)

Add optional @openai/codex-sdk exec-backed runner mode behind CODEX_RUNTIME=sdk, with CODEX_RUNTIME_SDK_ROLES canary limiting and app-server fallback for unsupported flows.

runners/codex-runner/test/runtime-mode.test.ts

@@ -0,0 +1,64 @@
+import { describe, expect, it } from 'vitest';
+
+import { resolveCodexRuntimeMode } from '../src/runtime-mode.js';
+
+describe('codex runner runtime mode selection', () => {
+  it('keeps app-server as the default runtime', () => {
+    expect(
+      resolveCodexRuntimeMode({}, { codexGoals: false }, 'do work'),
+    ).toEqual({
+      mode: 'app-server',
+      reason: 'default',
+    });
+  });
+
+  it('selects SDK mode when CODEX_RUNTIME=sdk', () => {
+    expect(
+      resolveCodexRuntimeMode(
+        { CODEX_RUNTIME: 'sdk' },
+        { codexGoals: false },
+        'do work',
+      ),
+    ).toEqual({ mode: 'sdk', reason: 'CODEX_RUNTIME=sdk' });
+  });
+
+  it('canary-limits SDK mode to configured paired roles', () => {
+    expect(
+      resolveCodexRuntimeMode(
+        { CODEX_RUNTIME: 'sdk', CODEX_RUNTIME_SDK_ROLES: 'owner,arbiter' },
+        { codexGoals: false, roomRole: 'owner' },
+        'do work',
+      ),
+    ).toEqual({ mode: 'sdk', reason: 'CODEX_RUNTIME=sdk' });
+    expect(
+      resolveCodexRuntimeMode(
+        { CODEX_RUNTIME: 'sdk', CODEX_RUNTIME_SDK_ROLES: 'owner,arbiter' },
+        { codexGoals: false, roomRole: 'reviewer' },
+        'do work',
+      ),
+    ).toEqual({ mode: 'app-server', reason: 'sdk-role-not-enabled' });
+  });
+
+  it('falls back to app-server for /compact because SDK lacks compaction', () => {
+    expect(
+      resolveCodexRuntimeMode(
+        { CODEX_RUNTIME: 'sdk' },
+        { codexGoals: false },
+        '/compact',
+      ),
+    ).toEqual({
+      mode: 'app-server',
+      reason: 'sdk-unsupported-session-command',
+    });
+  });
+
+  it('falls back to app-server when Codex goals are enabled', () => {
+    expect(
+      resolveCodexRuntimeMode(
+        { CODEX_RUNTIME: 'sdk' },
+        { codexGoals: true },
+        'do work',
+      ),
+    ).toEqual({ mode: 'app-server', reason: 'sdk-unsupported-goals' });
+  });
+});