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

TASKS.md

@@ -261,3 +261,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T23b | 1 | FAIL | source cards and web results with a handler were no Tab stops; in a narrow column the sources row and a passage's header ran past their edge |
 | T23b | 2 | FAIL | the gallery's narrow passage built a vec of one range, which clippy refuses, so the checks failed |
 | T23b | 3 | PASS | |
+| T23c | 1 | FAIL | suggestion arrows landed on disabled rows and left the cursor out of view; a dot ended a file mention; the drop veil never showed, since gpui styles a drag-over only on an element with a hitbox; a parameter's places came from its step alone; the gallery's prompt menu and Reset only logged |