feat(backends): add codex/gemini external runners and wire backend selection

src/config/schema.test.ts

@@ -196,6 +196,38 @@ describe('configSchema — server', () => {
   });
 });
 
+describe('configSchema — backends', () => {
+  const minimalConfig = {
+    telegram: { bot_token: 'test', allowed_chat_ids: [1] },
+    models: { default: { provider: 'anthropic', model: 'claude-3' } },
+  };
+
+  it('defaults backend flags to native enabled and externals disabled', () => {
+    const result = configSchema.parse(minimalConfig);
+    expect(result.backends.native.enabled).toBe(true);
+    expect(result.backends.claude_code.enabled).toBe(false);
+    expect(result.backends.opencode.enabled).toBe(false);
+    expect(result.backends.codex.enabled).toBe(false);
+    expect(result.backends.gemini.enabled).toBe(false);
+  });
+
+  it('accepts explicit external backend configs', () => {
+    const result = configSchema.parse({
+      ...minimalConfig,
+      backends: {
+        native: { enabled: false },
+        codex: { enabled: true, path: '/usr/local/bin/codex' },
+        gemini: { enabled: true, path: '/usr/local/bin/gemini' },
+      },
+    });
+    expect(result.backends.native.enabled).toBe(false);
+    expect(result.backends.codex.enabled).toBe(true);
+    expect(result.backends.codex.path).toBe('/usr/local/bin/codex');
+    expect(result.backends.gemini.enabled).toBe(true);
+    expect(result.backends.gemini.path).toBe('/usr/local/bin/gemini');
+  });
+});
+
 describe('configSchema — browser', () => {
   const minimalConfig = {
     telegram: { bot_token: 'test', allowed_chat_ids: [1] },