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/format.ts

@@ -15,3 +15,10 @@ export function fmtAge(ms: number): string {
   const h = Math.floor(m / 60);
   return h + 'h ' + (m % 60) + 'm';
 }
+
+/** Wall-clock HH:MM:SS (UTC) from a unix-seconds value — labels
+ * turn-start / turn-end rows in the live stream (ported from app.js's
+ * `fmtClock`). */
+export function fmtClock(sec: number): string {
+  return new Date(sec * 1000).toISOString().slice(11, 19);
+}