OwnershipNotice pseudo-field appears as sort option in collection list

[jamiefolsom]

The `_ownershipNotice` pseudo-field added by PR #586 for the read-only banner shows up as a sort option in the Posts and Paths collection list views:

• "_ownershipNotice (Ascending)"
• "_ownershipNotice (Descending)"

This is cosmetic noise. The field is a UI-only component (`OwnershipNotice.tsx`) that renders a banner, not actual data.

Possible fixes:

• Find a TinaCMS field option to exclude from sort/filter
• Use a different mechanism to inject the banner (e.g., `cmsCallback` DOM injection instead of a pseudo-field)
• Accept it as a minor cosmetic issue