Fix: Handle missing config file gracefully (#2)

[xyaz1313]

Fix for #2: Crash when config file missing

Problem

The app crashed with a FileNotFoundError when ~/.config/task-cli/config.yaml did not exist.

Solution

• load_config() now checks if the config file exists before trying to read it
• If missing, it creates the directory structure and writes a default config file
• Added DEFAULT_CONFIG constant with sensible defaults matching config.yaml.example
• Integrated load_config() into main() so config is loaded on every command

Testing

Added 3 new test cases:

• test_load_config_creates_default_when_missing — verifies default config creation
• test_load_config_reads_existing — verifies existing config is read correctly
• test_default_config_is_valid_yaml — validates the default config content

All 5 tests pass:

test_task.py::test_validate_description PASSED
test_task.py::test_validate_task_id PASSED
test_task.py::test_load_config_creates_default_when_missing PASSED
test_task.py::test_load_config_reads_existing PASSED
test_task.py::test_default_config_is_valid_yaml PASSED

Fixes #2