docs(base): clarify field description usage in json

[zgz2048]

Summary

• clarify in base field shortcut help text that description should be passed via --json.description
• update base field skill/reference docs to keep the existing --json model
• drop the extra field-specific flag and test/code-path changes from the earlier revision

Why

The existing field shortcuts already pass arbitrary JSON bodies through. This update only needs to make the description usage explicit while keeping the original command shape.

Validation

• go test ./shortcuts/base -run 'Test(DryRunFieldOps|BaseFieldExecuteUpdate|BaseFieldExecuteCRUD|BuildFieldBody)$'

[CLAassistant]

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