[iota] Add JSON output, safe config, and shared validators

[iota3-cell]

Summary

• Add --json support for add, list, and done.
• Create a default config file when ~/.config/task-cli/config.yaml is missing instead of raising FileNotFoundError.
• Add tests for JSON output and missing-config behavior.
• Update README with JSON examples and default config behavior.

Refactor

• Extract shared path helpers into utils/paths.py.
• Extract validation helpers into utils/validation.py.
• Remove duplicate validation/path helpers from command modules.

Verification

• python3 -m pytest /tmp/ai-gitops-target/test_task.py
• Manual CLI smoke test with temporary HOME for list --json, add --json, and done --json

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