Optional chat sorting by the latest delivered result #18

Following CONTRIBUTING.md's guidance for larger features: could we offer an optional chat sort order based on the most recent completed result, while keeping the existing workspace grouping and index-date order as the default?

With several parallel conversations, the session index can keep old rows in fixed positions even when they deliver a new result. Sorting by opening/focus time has a different usability problem: clicking a row makes it jump away from the pointer. The proposed rule is deliberately stable: only a new explicit task completion changes delivery recency. Opening, focusing, reading, starting work, questions, and streamed tokens do not move rows.

An independent implementation is ready for review: codexExtras.chatSortOrder with recent (default) and latestResult. It caches bounded log-tail reads, remembers observed completion timestamps locally across reloads, and refreshes through file events. It needs no live IPC, status icons, or source transcript modifications. Completions outside the bounded tail can be unknown on first use; once observed, they are remembered. No version bump is proposed.