agent: LiveStream/Row/DetailsRow — Preact live event pane

Ports the row taxonomy in docs/terminal-rendering.md (app.js's
renderStream/renderRichToolUse/renderToolResult/renderTaskEvent) to
real Preact components + hooks:

- lib/streamRow.ts: plain-data StreamRow model (one row = one line/
  panel), lib/classifyEvent.ts: raw stream-json event -> StreamRow[],
  almost entirely dispatching on the backend's precomputed _icon/
  _summary/_category/_body/_body_type fields, same as the old client.
- lib/markdown.ts, lib/linkify.tsx: sanitized-markdown + auto-link
  helpers, same behavior as app.js's mdNode/terminal.js's linkify.
- hooks/useLiveStream.ts: backfill + SSE + seq-dedupe + coalescing,
  reduced to a plain StreamRow[] — deliberately has no opinion on
  scroll position, only on what rows exist.
- components/Row.tsx: renders one StreamRow (flat or details).
- components/LiveStream.tsx: owns the scrollable DOM node + a from-
  scratch sticky-bottom implementation — not the old MutationObserver
  + rAF snap-animation + smoothScrollingUntil gate. stickToBottom is
  plain state driven by the scroll handler; snapping is an instant
  scrollTop write in a useLayoutEffect that runs after Preact has
  already committed the new rows, so it always sees the real
  scrollHeight. A load-older prepend is told apart from a normal
  append via an explicit ref set right before calling loadMore(),
  rather than inferred from DOM mutation shape after the fact.

Wired into Root.tsx below Header/StatusChips; turn_start/turn_end
also nudge useAgentState's refresh() for a snappier badge update than
the plain poll interval.

Reuses @hive/shared/terminal.css's existing row-kind classes as-is —
the taxonomy's visual language isn't what changed, the component
model underneath it is.

frontend/packages/agent/src/lib/linkify.tsx

@@ -0,0 +1,39 @@
+// Auto-linkify bare http(s) URLs in plain (non-markdown) row text —
+// Preact-node port of @hive/shared/terminal.js's `linkify` (text-node
+// output there, JSX fragment here). Same regex + trailing-punctuation
+// strip; deliberately text-only, never innerHTML, so untrusted row text
+// (matrix-relayed bodies, tool args) can't inject markup this way.
+import type { JSX } from 'preact';
+
+const LINKIFY_URL_RE = /https?:\/\/[^\s<>"']+/g;
+
+export function linkifyToNodes(text: string | null | undefined): (string | JSX.Element)[] {
+  const str = text == null ? '' : String(text);
+  if (str.indexOf('://') === -1) return [str];
+  const out: (string | JSX.Element)[] = [];
+  let last = 0;
+  let m: RegExpExecArray | null;
+  let key = 0;
+  LINKIFY_URL_RE.lastIndex = 0;
+  while ((m = LINKIFY_URL_RE.exec(str)) !== null) {
+    let url = m[0];
+    const trail = url.match(/[.,;:!?)\]}'"]+$/);
+    const tail = trail ? trail[0] : '';
+    if (tail) url = url.slice(0, -tail.length);
+    if (m.index > last) out.push(str.slice(last, m.index));
+    if (!url.slice(url.indexOf('://') + 3)) {
+      // Nothing past the scheme — not a real URL, emit verbatim.
+      out.push(m[0]);
+    } else {
+      out.push(
+        <a key={key++} href={url} target="_blank" rel="noopener noreferrer">
+          {url}
+        </a>,
+      );
+      if (tail) out.push(tail);
+    }
+    last = m.index + m[0].length;
+  }
+  if (last < str.length) out.push(str.slice(last));
+  return out;
+}