license field: add an example showing a machine-readable training-rights reservation

Disclosure up front: I work on SkillRights (https://skillrights.org), an open licence family for agent skills, which is why I care about this.

The license field guidance currently shows one example ("Proprietary. LICENSE.txt has complete terms"). Would you consider a second documented example showing an SPDX LicenseRef identifier, the shape authors use when they want to permit execution while reserving model-training rights?

license: LicenseRef-SkillRights-NoTrain-1.0

Per SPDX LicenseRef practice the licence text ships beside the skill in a LICENSES/ directory, which also fits the field's existing "reference to a bundled license file" guidance. Any LicenseRef identifier works the same way; ours is one example of the pattern, not a request for special treatment. No spec change requested, only an example. Happy to PR the docs line if that is easier.