## Summary
Adds a line-reference and annotation system for diffs in the Agents UI.
Users can click line numbers in the Git diff panel to open an inline
prompt input, type a comment, and have a reference chip + text added to
the chat message input.
## Changes
### Backend
- Added `diff-comment` type to `ChatInputPart` and `ChatMessagePart` in
`codersdk/chats.go` with `FileName`, `StartLine`, `EndLine`, `Side`
fields
### Frontend
- **`DiffCommentContext`**: React context/provider managing pending diff
comments with `addReference`, `removeComment`, `restoreComment`,
`clearComments`
- **`DiffCommentNode`**: Lexical `DecoratorNode` rendering inline chips
in the chat input showing file:line references. Chips are clickable
(scroll to line in diff), removable, and support undo/redo via mutation
tracking
- **`InlinePromptInput`**: Textarea annotation rendered inline under
clicked lines in the diff. Supports multiline (Shift+Enter), submit
(Enter), cancel (Escape)
- **`FilesChangedPanel`**: Line click/drag-select handlers open the
inline input. On submit, a badge chip + plain text are inserted into the
Lexical editor
- **`AgentDetail`**: Bidirectional sync between DiffCommentContext and
Lexical editor. Comments are sent as `diff-comment` parts on message
submit
- **`ConversationTimeline`**: Renders `diff-comment` message parts with
file:line labels
## How it works
1. Click a line number in the diff → inline textarea appears below that
line
2. Type a comment and press Enter → reference chip appears in chat input
with your text after it
3. Send the message → diff-comment parts are included alongside the
message text
Kyle Carberry
2ad0e74e67