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/streamRow.ts

@@ -0,0 +1,49 @@
+// Row model for the live event stream. One `StreamRow` = one rendered
+// line/panel in the terminal pane; `classifyEvent` (classifyEvent.ts)
+// turns a raw harness/stream-json event into zero or more of these, and
+// `<Row>` (components/Row.tsx) renders one. Kept as plain data (not JSX)
+// so the append/coalesce bookkeeping in useLiveStream.ts stays pure —
+// see docs/terminal-rendering.md for the exact taxonomy this mirrors.
+
+export interface StreamRowMeta {
+  cls: string;
+  text: string;
+}
+
+export interface StreamRow {
+  /** Stable across re-renders; reused in place when a row is coalesced
+   * (e.g. the thinking-token counter) so Preact updates rather than
+   * remounts it. */
+  key: string;
+  /** Space-joined class names appended after "row" / "row details", e.g.
+   * "turn-start", "tool-result error" — taken verbatim from
+   * @hive/shared/terminal.css's row-kind taxonomy. */
+  cssClass: string;
+  icon?: string;
+  fromHistory: boolean;
+  /** When set, an event that maps to the same coalesceKey and lands
+   * while this row is still the last one in the list replaces it in
+   * place instead of appending a new row (thinking-token ticks, status
+   * ticks, plugin_install started→completed). */
+  coalesceKey?: string;
+  /** `false`/absent → flat `<div class="row">`; `true` → `<details
+   * class="row">` with `.summary-text` + optional body. */
+  details?: boolean;
+  defaultOpen?: boolean;
+  /** Flat-row prefix text, or the details `<summary>` text. Linkified,
+   * never markdown. */
+  text?: string;
+  /** Small trailing spans after the prefix text (turn-time, unread
+   * count) — flat rows only. */
+  meta?: StreamRowMeta[];
+  /** Plain-text child block under a flat row (just the turn-start wake
+   * body today) — its own class, no markdown parsing. */
+  childText?: StreamRowMeta;
+  /** Sanitized-markdown body: appended under a flat row (assistant
+   * text) or inside an open details row (send/ask/answer/recv bodies). */
+  markdownBody?: string;
+  /** Plain `<pre>` body inside a details row (generic long tool output). */
+  plainBody?: string;
+  /** `+`/`-`/context diff body inside a details row (Edit tool). */
+  diffBody?: string;
+}