✨ add custom HTTP headers support via OCO_API_CUSTOM_HEADERS

Add OCO_API_CUSTOM_HEADERS variable to README, config enum,
and env parsing to allow JSON string of custom headers.
Validate that custom headers are valid JSON in config validator.
Extend AiEngineConfig with customHeaders and pass headers to
OllamaEngine and OpenAiEngine clients when creating requests.
Parse custom headers in utils/engine and warn on invalid format.
Add unit tests to ensure OCO_API_CUSTOM_HEADERS is handled
correctly and merged from env over global config.

This enables users to send additional headers such as
Authorization or tracing headers with LLM API calls.

test/unit/config.test.ts

@@ -122,6 +122,30 @@ describe('config', () => {
       expect(config.OCO_ONE_LINE_COMMIT).toEqual(false);
       expect(config.OCO_OMIT_SCOPE).toEqual(true);
     });
+    
+    it('should handle custom HTTP headers correctly', async () => {
+      globalConfigFile = await generateConfig('.opencommit', {
+        OCO_API_CUSTOM_HEADERS: '{"X-Global-Header": "global-value"}'
+      });
+
+      envConfigFile = await generateConfig('.env', {
+        OCO_API_CUSTOM_HEADERS: '{"Authorization": "Bearer token123", "X-Custom-Header": "test-value"}'
+      });
+
+      const config = getConfig({
+        globalPath: globalConfigFile.filePath,
+        envPath: envConfigFile.filePath
+      });
+
+      expect(config).not.toEqual(null);
+      expect(config.OCO_API_CUSTOM_HEADERS).toEqual('{"Authorization": "Bearer token123", "X-Custom-Header": "test-value"}');
+      
+      // Verify that the JSON can be parsed correctly
+      const parsedHeaders = JSON.parse(config.OCO_API_CUSTOM_HEADERS);
+      expect(parsedHeaders).toHaveProperty('Authorization', 'Bearer token123');
+      expect(parsedHeaders).toHaveProperty('X-Custom-Header', 'test-value');
+      expect(parsedHeaders).not.toHaveProperty('X-Global-Header');
+    });
 
     it('should handle empty local config correctly', async () => {
       globalConfigFile = await generateConfig('.opencommit', {