Reader: let the prompt scroll away

The prompt band was `position: sticky` at the top of the conversation scroller,
on the theory that deep inside a long turn the thing you want overhead is what
you asked for. In a pane-sized reader that is not how it reads: the message
looks like it is refusing to scroll, and on a phone — where a band of wrapped
prompt text can be a third of the viewport — it is most of the screen held back
from the thing you are scrolling toward.

Reported from the deployed branch on a phone. It scrolls with everything else
now; the tint and the hairlines are what mark it, which is what the base rule
was already for.

The opaque background override went with it. It existed only so scrolling
content would not show through a floating band, and over .cxv-body's --panel the
base `color-mix(--accent 7%, transparent)` composites to exactly the same
colour — so the whole `.cxv-col .cx-prompt` rule is gone rather than trimmed.

Removed everywhere, not just under the phone breakpoint: the reason it reads
badly in a pane is the pane, not the screen size.

tsc clean, exchanges ok, production build succeeds.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

web/src/conversation.css

@@ -174,12 +174,14 @@ mark.cx-hit.on { background: var(--accent); color: var(--panel); }
 .cxv-foot .cxv-path { min-width: 0; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
 .cxv-foot .spacer { flex: 1; min-width: 4px; }
 
-/* the prompt band sticks: deep inside a long turn, the thing you want overhead
-   is what you asked for — not a row of numbers */
-.cxv-col .cx-prompt {
-  position: sticky; top: 0; z-index: 2;
-  background: color-mix(in srgb, var(--accent) 7%, var(--panel));
-}
+/* The prompt band used to stick to the top of this scroller, on the theory that
+   deep inside a long turn you want what you asked for overhead. In a pane-sized
+   reader it reads as the message refusing to scroll away — and on a phone,
+   where the band can be a third of what you can see, that is most of the screen
+   held back. It scrolls with everything else now; the tint is what marks it.
+   The opaque background that came with it went too: it existed so scrolling
+   content would not show through a floating band, and over .cxv-body's --panel
+   the base transparent tint composites to the same colour. */
 
 /* ---------- phone ---------- */
 @media (max-width: 720px) {