bug: show a clear error when tasks.json is invalid

[MrParamecium]

Problem

The task tracker crashes with a Python traceback when tasks.json contains invalid JSON.

Steps to reproduce

1. Replace the contents of tasks.json with invalid JSON:

   [
     {"title": "Broken task"

2. Run:

   python3 task_tracker.py list

Actual behavior

The program crashes and displays a Python JSONDecodeError traceback.

Expected behavior

The program should display a clear error message:

tasks.json contains invalid JSON

It should exit without modifying or replacing the original file.

Acceptance criteria

• list, add, and complete handle invalid JSON consistently
• no Python traceback is displayed
• the original tasks.json remains unchanged
• automated tests cover invalid JSON