Task commands now return structured, typed errors instead of the legacy
exit-code envelope: every failure carries a stable category, subtype, and
recovery hint, so callers can branch on the error class instead of parsing
messages. Exit codes derive from the error category — input validation exits 2,
a permission denial exits 3, other API errors exit 1.
Batch operations (adding tasks to a tasklist, creating a tasklist with tasks)
now report partial failure honestly: the per-item successes and failures stay
on stdout and the command exits non-zero instead of masking failures as a
success.
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