hive-agent: add stream_enrich module — stamp _icon/_summary/_category on SSE events

Move per-message rendering logic from frontend JS to backend Rust.
A new stream_enrich::enrich() function stamps display fields onto
LiveEvent::Stream payloads at SSE-emit time (both live tail and history
replay), so the frontend can consume pre-computed fields instead of
re-implementing the dispatch logic in JavaScript.

Phase 1: backend stamps _icon/_summary/_category; client falls back to
its own JS tables when absent. Zero user-visible change.

- system events: _category (drop/thinking_tok/note/details) + _summary
  and optional _body (commands_changed expands to a slash-cmd list)
- assistant tool_use entries: _icon + _summary per tool; rich tools
  (Write, Edit, send, ask, answer) get _category: 'rich'
- enrichment applied in web_ui/stream.rs at emit time; DB stores raw
  events so no migration is needed when enrichment logic changes
- idempotent: existing _-prefixed fields are left unchanged

hive-agent/src/web_ui/stream.rs

@@ -40,6 +40,17 @@ pub(super) async fn events_history(
     };
 
     let (events, min_id, has_more) = state.bus.history_page(before, limit);
+    // Apply the same enrichment as the live SSE path so history replay
+    // and live tail deliver identical shapes. The DB stores raw events.
+    let events: Vec<_> = events
+        .into_iter()
+        .map(|mut se| {
+            if let crate::events::LiveEvent::Stream(ref mut v) = se.event {
+                crate::stream_enrich::enrich(v);
+            }
+            se
+        })
+        .collect();
     let mut resp = serde_json::json!({
         "events": events,
         "min_id": min_id,
@@ -68,7 +79,15 @@ pub(super) async fn events_stream(
         .unwrap_or_default(),
     );
     let live = BroadcastStream::new(rx).filter_map(|res| {
-        let ev = res.ok()?;
+        let mut ev = res.ok()?;
+        // Enrich stream-json values with pre-computed display fields
+        // (`_icon`, `_summary`, `_category`) so the frontend doesn't need to
+        // duplicate the dispatch logic. The DB stores raw events; enrichment
+        // is applied here so both the live tail and the history endpoint
+        // deliver the same shape (see `events_history` below).
+        if let crate::events::LiveEvent::Stream(ref mut v) = ev.event {
+            crate::stream_enrich::enrich(v);
+        }
         let json = serde_json::to_string(&ev).ok()?;
         Some(Ok(Event::default().data(json)))
     });