T24c: Agent, changes

- FileChangeCard: a proposed change to one file, its path and counts,
  Reject and Accept in a slot that keeps presses off the header; opened,
  the diff through a headless git::DiffViewer with its folds opening
- MultiFileDiffReview: every file at once, the total counts, how many are
  reviewed, Reject all and Accept all; verdicts as collab::Decision
- git::DiffViewer gains headless, leaving the file's header to its owner
- CommandExecutionCard is terminal::CommandBlock
- gallery: three proposed files and a command's block

src/chat/mod.rs

@@ -28,6 +28,7 @@ pub use code::CodeBlock;
 pub use composer::{AttachmentButton, DragDropOverlay, InputHint, PromptInput, SendButton};
 pub use conversations::{Conversation, ConversationItem, ConversationList, new_chat_button};
 pub use list::{DateSeparator, MessageList, ScrollToBottomButton};
+pub(crate) use media::file_icon;
 pub use media::{FileMessage, ImageGrid, ImageMessage, LinkPreviewCard};
 pub use message::{
     ChatContainer, MessageAvatar, MessageBubble, MessageFooter, MessageHeader, Role,