T23a: AI chat, messages

- messages: yours in a quiet bubble, the assistant's as prose beside its mark,
  system notes, errors; avatars, headers with model and time, footers
- MessageList over gpui's bottom-aligned list: drawn near the view, held at the
  newest, counting arrivals while away; a way back down; day separators
- streaming text and markdown reveal toward the text as it grows, with a caret;
  a blinking cursor before the first words
- CodeBlock with language, copy, run, apply, download and folding;
  MarkdownRenderer::code lets messages draw fences with it
- pictures, grids, files, voice notes, video over host frames, link previews
- quotes, actions, an editor that takes focus, versions, regenerate, stop and
  continue
- errors with retry, a rate limit counting down, feedback, thinking blocks
- gallery: the AI Chat page with scripted states

AGENTS.md

@@ -114,6 +114,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - A find box is `editor::FindWidget`, over code, a terminal or a document's pages. It shows the replace fold and the option toggles only when the owner handles them.
 - Taffy 0.9 hands negative free space to auto margins and centering alike, so a centered child wider than its scroll box starts past the scroll's reach. Center only what fits the measured box (`documents::DocumentViewer`).
 - A mode that leaves on Escape holds its own focus through `primitives::hold_focus`: turned on while focus is elsewhere, it takes focus so Escape reaches it, and `hand_back` returns it (`documents::ZenMode`, `collab::FollowMode`).
+- A chat holds at its newest message through gpui's `list` with `ListAlignment::Bottom`: it sticks while scrolled to the end, and `scroll_to` one past the last item sticks it again; that call sends no scroll event, so the owner clears its own away state (`chat::MessageList`).
 - A mark over a text field reads the field at paint time, from a canvas laid over it (`collab::RemoteCursor`): by then `TextInput::bounds_for` holds this frame's layout, and a still page never renders again to catch up.
 - A callback field holds a named `type` alias; clippy's type_complexity rejects `Option<Rc<dyn Fn(..)>>`. A chapter shares one alias per shape.
 - A component that fills its container says `w_full` on its root. As a window's root, or in a flex row, it would shrink to its text's narrowest width.