fix: handle missing config file gracefully (#2)

[ttntk]

What

Fixes #2

load_config() no longer crashes with FileNotFoundError when ~/.config/task-cli/config.yaml is missing. It creates a default config automatically on first run.

How

• Added get_config_path() helper for testability
• load_config() checks config_path.exists() first
• If missing: creates parent dirs + writes DEFAULT_CONFIG
• If present: reads existing content (unchanged behavior)
• Added 4 pytest-style tests: missing file, existing file, nested dirs, no-crash

Testing

python -m pytest test_task.py -v
6 passed

All existing tests pass. No unrelated changes.