feat: add json output and default config

[criticalconnections]

Summary

• Adds a --json flag to add, list, and done commands.
• Emits parseable JSON for successful command output and empty/error states.
• Creates a default config file when ~/.config/task-cli/config.yaml is missing instead of crashing.
• Adds tests for default config creation and JSON output.

Fixes #1.
Fixes #2.

/claim #1
/claim #2

Validation

• python -m pytest test_task.py
• python -m compileall task.py commands test_task.py
• git diff --check