fix: gracefully handle missing config file with defaults

[brutusworker-arch]

Summary

• Fix crash (FileNotFoundError) when ~/.config/task-cli/config.yaml is missing
• load_config() now creates the config directory and writes a sensible default config file when none exists

Changes

• Added DEFAULT_CONFIG constant with default YAML configuration
• Added get_config_path() helper for testability
• Modified load_config() to check for config file existence before opening, creating defaults if missing
• Added two tests: one for missing config (verifies default creation) and one for existing config (verifies it reads correctly)

Testing

1. Delete config: rm -f ~/.config/task-cli/config.yaml
2. Run: python task.py list — should work without crash
3. Verify default config created: cat ~/.config/task-cli/config.yaml
4. Run tests: python -m pytest test_task.py -v

Closes #2

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Closes #2

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