terminal: anchor tail pill outside .terminal-wrap stacking context (#375)

#375 set `z-index: 35` on the agent page's tail pill so it'd float
above the fixed composer (z-index 30). The fix only landed because
nothing else in `.agent-main` created a stacking context. But the
pill is anchored inside `.terminal-wrap`, which carries
`backdrop-filter: blur(...) saturate(...)` for the frost effect —
and `backdrop-filter` CREATES A STACKING CONTEXT. The pill's
z-index 35 was trapped inside that context and never got to compete
with the composer's z-index 30 in the root stacking context, so the
operator still saw the badge clipped under the input box.

Same root cause on the flow page — `.flow-main .terminal-wrap`
inherits the same backdrop-filter rule.

Fix: anchor the pill in `.agent-main` / `.flow-main` instead of
`log.parentElement` (= `.terminal-wrap`). Both ancestors are
`position: absolute` with `overflow: hidden` but NO backdrop-filter
or other stacking-context creators, so the pill's z-index reaches
the root and properly floats above the composer.

Geometry unchanged — `.agent-main` / `.flow-main` and the
`.terminal-wrap` they contain both `inset: 0` the same area, so the
pill's `bottom: calc(--composer-h + 0.6em)` lands at the same y.

Also added `.flow-main .tail-pill { z-index: 35 }` (the flow page
was missing the per-page z-index bump that the agent page already
had).

`pillAnchor` is an existing opt in @hive/shared/terminal.js (the
default is `log.parentElement`); both consumers now set it explicitly.

frontend/packages/agent/src/app.js

@@ -1143,6 +1143,18 @@ window.marked = marked;
 
     const term = HiveTerminal.create({
       logEl: log,
+      // Anchor the `↓ N new` pill in `.agent-main` (NOT the default
+      // `log.parentElement` = `.terminal-wrap`). `.terminal-wrap`
+      // applies `backdrop-filter` for the frost effect, which
+      // creates a CSS stacking context — the pill's z-index 35
+      // (agent.css #375) then gets TRAPPED inside that context and
+      // can't compete with the fixed composer's z-index 30 in the
+      // root stacking context. Anchoring in `.agent-main` (no
+      // backdrop-filter, no other stacking-context creators) lets
+      // the pill's z-index reach the root and properly float above
+      // the composer. Geometry unchanged — `.agent-main` and
+      // `.terminal-wrap` both `inset: 0` fill the same area.
+      pillAnchor: $('agent-main'),
       historyUrl: '/events/history',
       streamUrl: '/events/stream',
       renderers: {

frontend/packages/dashboard/src/app.js

@@ -2277,8 +2277,18 @@ window.marked = marked;
       // Register this row so future replies can reference it.
       if (ev.id != null && ev.id > 0) msgRowMap.set(ev.id, row);
     }
+    // Anchor the `↓ N new` pill in `.flow-main` (NOT the default
+    // `log.parentElement` = `.terminal-wrap`). `.terminal-wrap`
+    // applies `backdrop-filter`, which creates a CSS stacking
+    // context — the pill's z-index would otherwise be trapped
+    // inside and clipped under the fixed composer (issue #375).
+    // `.flow-main` has no backdrop-filter / stacking-context
+    // creators, so the pill's z-index reaches the root and floats
+    // above the composer.
+    const flowMain = document.querySelector('.flow-main');
     HiveTerminal.create({
       logEl: flow,
+      pillAnchor: flowMain,
       historyUrl: '/dashboard/history',
       streamUrl: '/dashboard/stream',
       renderers: {

frontend/packages/dashboard/src/dashboard.css

@@ -1421,8 +1421,15 @@ body.flow-shell .tabbar .tab.active.tab-link {
   scroll-padding-bottom: calc(var(--flow-composer-h) + 0.8em);
   overflow: auto;
 }
+/* Tail pill (↓ N new): bottom offset clears the floating composer.
+   z-index escapes the stacking context the .terminal-wrap's
+   backdrop-filter creates (issue #375) — app.js anchors the pill
+   on .flow-main now (not .terminal-wrap), so this z-index reaches
+   the root stacking context and properly floats above the
+   composer at z-index 30. */
 .flow-main .tail-pill {
   bottom: calc(var(--flow-composer-h) + 0.6em);
+  z-index: 35;
 }
 
 .flow-composer {