openai/gpt-5.2-chat → A (3:2)

Side A introduces a new `url_rules` engine and refactors `ItemId` to use canonical full URLs (e.g., `https://reddit.com/...`) across parsing, storage, parents, breadcrumbs, projection, and Reddit integration—removing the old ad-hoc `canonicalize` logic and centralizing normalization. This is a foundational identity-layer redesign affecting many modules and tests. Side B adds a valuable GitHub resolver module and fixes the external garden index, but it builds on existing structures rather than reshaping core URL identity semantics.