feat: merge hidden-user turns and move message metadata to turn footer

- Remove assistant message headers; show provider icon, model, agent,
  thinking variant, duration and time in the turn footer (metadata left,
  hover-revealed actions right)
- Merge turns started by hidden user messages (subagent nudges) into the
  previous turn so Activity, footer and spacing stay continuous
- Treat compaction summary text (info.summary) as justification activity
  in sorted mode and skip it when picking the turn summary
- Interleave activity segments with standalone tool rows so Agent Task
  sits chronologically between activity sections

packages/ui/src/components/chat/message/hiddenUserMessage.ts

@@ -0,0 +1,34 @@
+import type { Message, Part } from '@opencode-ai/sdk/v2';
+
+import { deriveMessageRole } from './messageRole';
+import { filterVisibleParts, normalizeParts } from './partUtils';
+import { normalizeUserDisplayParts } from './normalizeUserDisplayParts';
+
+/**
+ * A user message is hidden when none of its parts survive display
+ * normalization (e.g. synthetic subagent-completion nudges). Turns separated
+ * only by such messages should render as one continuous flow.
+ */
+// Streaming recomputes turn projections often; cache by parts reference so
+// unchanged messages resolve without re-running display normalization.
+const hiddenByPartsPlanMode = new WeakMap<Part[], boolean>();
+const hiddenByPartsNoPlanMode = new WeakMap<Part[], boolean>();
+
+export const isHiddenUserMessage = (
+    entry: { info: Message; parts: Part[] } | null | undefined,
+    options: { planModeEnabled: boolean }
+): boolean => {
+    if (!entry) return false;
+    if (!deriveMessageRole(entry.info).isUser) return false;
+
+    const cache = options.planModeEnabled ? hiddenByPartsPlanMode : hiddenByPartsNoPlanMode;
+    const cached = cache.get(entry.parts);
+    if (cached !== undefined) {
+        return cached;
+    }
+
+    const parts = normalizeUserDisplayParts(normalizeParts(entry.parts), { planModeEnabled: options.planModeEnabled });
+    const hidden = filterVisibleParts(parts, { includeReasoning: true }).length === 0;
+    cache.set(entry.parts, hidden);
+    return hidden;
+};