fix: create default config when missing

[autochamchikim-pixel]

Summary

• Makes load_config() create ~/.config/task-cli/config.yaml with sensible defaults when it is missing.
• Calls the config loader before command dispatch so every command handles the missing-config case consistently.
• Adds regression coverage for direct config loading and CLI execution with an isolated HOME.
• Updates README configuration notes.

Test Plan

• uvx --from pytest pytest -q test_task.py

/claim #2