[codex] Clarify OneSignal user ID attribute

[anglinb]

Summary

• Clarify that IntegrationAttribute.onesignalId should contain the OneSignal User ID.
• Add the changelog note for the SDK documentation clarification.

Validation

• Ran staged whitespace checks with git diff --cached --check.
• Not run: builds, tests, or lint per repo preference.

[greptile-apps]

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