T23c round 1 fix

- suggestions: arrows step past disabled rows and scroll the cursor into view;
  the cursor starts on an enabled row; Enter refuses a disabled one
- active_trigger takes the query's word test: one word for mentions, any
  non-space for the composer, so a file name keeps its dot
- DragDropOverlay takes the drop itself, which gives it the hitbox gpui needs
  to style a drag-over
- ParameterPanel shows the places of the step, value and both ends
- gallery: the prompt menu fills the draft; Reset restores the system prompt
- AGENTS: drag-over styles need a hitbox; debug_bounds never clear

AGENTS.md

@@ -129,6 +129,8 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - Clippy's `single_range_in_vec_init` rejects `[a..b]` and `vec![a..b]`; bind the range first.
 - A component built on `lists::Tree` fills its box; the host gives it a height. A fixed-width column beside a `flex_1` one takes `flex_none`, or it shrinks.
 - Editor rows read their text through `CodeEditor::row_text`: a multi-line ghost cuts its line, and the rest follows the ghost's last line.
+- gpui styles a drag-over only on an element with its own hitbox. A veil that shows while files hover takes the drop itself (`chat::DragDropOverlay`).
+- gpui 0.2.2 never clears `debug_bounds`: a test sees an element appear, not leave.
 
 ## Decisions