Fix missing config handling

[darshan-Jahagirdar]

Fixes #2.

Changes

• Create ~/.config/task-cli/config.yaml with default settings when it is missing.
• Load config before command execution so real CLI commands exercise the missing-config path.
• Preserve existing config files unchanged.
• Document the automatic default config behavior.

Tests

• python -m pytest test_task.py -> 5 passed
• python task.py list with a temporary empty home directory -> creates config and prints No tasks yet!

/claim #2

[darshan-Jahagirdar]

Validation update for this PR:

ext py -3.11 -B -m pytest test_task.py git diff --check

Result: 5 tests passed and the diff whitespace check passed. I installed pytest into a temporary test-only directory for this run and removed it afterward.