Add `core/get-user` ability

[jorgefilipecosta]

Introduce a new Abilities API ability, core/get-user, to retrieve profile data for a specific user by id, username, or email.

Core currently has core/get-user-info for the current authenticated user only (and not exposed in REST). Agent/workflows use cases need to be able to look up any user when permissions allow it.

Proposed behavior

• Register core/get-user in the user ability category.
• Input schema:
  • Require one of: id (integer), username (string), or email (string).
  • Optional: include_capabilities (boolean, default false).
• Output schema includes:
  • id, username, email, display_name, first_name, last_name, nickname, description, url, link, slug, registered_date, roles, locale, avatar_urls.
  • Optional capabilities when include_capabilities is true.
• Permission model:
  • User must be logged in.
  • Users can view their own record.
  • Viewing other users requires list_users.
• Expose the ability in REST (show_in_rest true).
• Return user_not_found error when no matching user exists.

Notes / follow-ups

• This ticket scopes the initial profile payload.
• Follow-up can extend response data (for example, registered user meta with schema).

References

• ​https://github.com/WordPress/wordpress-develop/pull/10775
• ​https://github.com/WordPress/ai/issues/40