Add get and post/pages abilities

[jorgefilipecosta]

This tracks the addition of get abilities for post types via the Abilities API, enabling AI agents to retrieve posts and pages through core/post-type/{slug}/get. It should introduce a show_in_abilities option on register_post_type() to control which post types expose abilities, enabled by default for post and page initially (then we can expand to menus, media etc)-
It should support single-post retrieval by ID and multi-post querying with filters for status, author, search, taxonomy, meta, date, comment status, ping status, advanced taxonomies, date or meta queries.