feat: implement JSON task export

[NotMattyS]

Description

While our custom fault-tolerant .tsk format is great, we want to allow users to export their tasks into a universal JSON format so they can use their data elsewhere. We need an export <path> command.

Proposed Behavior

Typing export my_tasks.json should iterate over the current tasks and write a valid JSON array into the specified path.

Implementation Details

1. Engine Method: Add void export_to_json(const std::string &path); to task_manager.
2. Lightweight Serialization: To keep the project free of heavy external dependencies, manually serialize tasks into JSON string loop using std::ofstream and std::stringstream. The output format should look like this:

   [
       {
           "id": 1,
           "name": "Task Name",
           "description": "Task Description",
           "completed": false
       }
   ]

3. Command & Parser:
   • Create an export_json_command that receives the path string.
   • Bind it to parse_cli_arguments handling arguments check (requires exactly 2 arguments: export and <path>).

Acceptance Criteria

• export <path> creates a valid JSON file.
• Exported JSON contains all task currently loaded in memory.
• Strings are escaped correctly.
  At minimum, escape quotes, backslashes and control characters.
• File open failures are handled gracefully.
• No external JSON library is added.

Notes

• If the file cannot be opened, print error message and abort export.
• Do not modify the internal .tsk persistence format.
• It is recommended to use a helper such as:

  std::string escape_json_string(const std::string &input);