Emit deprecation warning for `Task.error` only once per process

[FernandoCelmer]

Labels: enhancement, good first issue

Context

dotflow/core/task.py:217-243 emits a DeprecationWarning on every get and every set of the deprecated Task.error property. Legacy consumers reading error in a hot path pay the cost on each access. Test runs with -W error (which converts warnings to exceptions) fail spuriously when a downstream library still references the old attribute.

Concept

Warn once per process. Use a module-level flag.

Proposed change

# dotflow/core/task.py
_ERROR_DEPRECATION_WARNED = False

@property
def error(self):
    global _ERROR_DEPRECATION_WARNED
    if not _ERROR_DEPRECATION_WARNED:
        warnings.warn(
            "'error' is deprecated, use 'errors' instead. "
            "Note: 'TaskError.exception' is now a string "
            "(e.g. 'ValueError'), not an Exception object.",
            DeprecationWarning,
            stacklevel=2,
        )
        _ERROR_DEPRECATION_WARNED = True
    if not self.errors:
        return TaskError()
    return self.errors[-1]

Apply the same pattern to the setter.

Acceptance criteria

• Both error getter and error setter use the flag
• Test: Task.error accessed N times → pytest.warns catches
  exactly one warning
• Document removal target version in the docstring

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