✨ 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.

README.md

@@ -109,6 +109,7 @@ Create a `.env` file and add OpenCommit config variables there like this:
 OCO_AI_PROVIDER=<openai (default), anthropic, azure, ollama, gemini, flowise, deepseek>
 OCO_API_KEY=<your OpenAI API token> // or other LLM provider API token
 OCO_API_URL=<may be used to set proxy path to OpenAI api>
+OCO_API_CUSTOM_HEADERS=<JSON string of custom HTTP headers to include in API requests>
 OCO_TOKENS_MAX_INPUT=<max model token limit (default: 4096)>
 OCO_TOKENS_MAX_OUTPUT=<max response tokens (default: 500)>
 OCO_DESCRIPTION=<postface a message with ~3 sentences description of the changes>