feat(dashboard): operator inbox with mark-as-read on Y3R C4LL

Agents that `send(to: "operator")` were easy to miss — they only
surfaced on the FL0W firehose with no read-state (#1469). Surface them
on the Y3R C4LL ("things waiting on you") tab as a proper inbox.

Backend:
- broker: `unread_for_recipient(recipient, limit)` — unacked messages
  for a recipient, newest-first. Mirrors `mark_all_read`'s filter
  EXACTLY (`recipient = ?1 AND acked_at IS NULL`, no `delivered_at`
  condition) so everything listed is exactly what mark-read clears —
  operator rows never get `delivered_at` set (no agent-socket recv).
- dashboard: `GET /api/operator-inbox` → `{ messages: [...] }` (id,
  from, body, at, in_reply_to, validated file_refs). Mark-read reuses
  the existing `POST /api/agent/operator/mark-all-read` (the route
  format-validates the name; "operator" passes; `mark_all_read`
  already acks `to="operator"` rows).

Frontend (Y3R C4LL):
- New ◆ 1NB0X ◆ section listing unread messages (sender · time · body,
  path-linkified) + a "✓ mark all read" button.
- Cold-loaded on page load + on tab activation; appended live from the
  broker `sent` stream (deduped on row id); cleared on mark-all-read.
- Unread count folds into the Y3R C4LL tab pill + the browser-title
  `(N)` prefix, so messages are visible from any tab.

Removing the now-redundant FL0W operator-inbox UI is a clean follow-up
(deferred to avoid a flow.js conflict with the in-flight #1473).
Backend (broker + route) is host-side — @damocles to review per plan.

Closes #1469.

hive-c0re/src/dashboard.rs

@@ -67,6 +67,7 @@ pub async fn serve(port: u16, coord: Arc<Coordinator>) -> Result<()> {
         .route("/api/approval-diff/{id}", get(get_approval_diff))
         .route("/api/state-file", get(get_state_file))
         .route("/api/reminders", get(api_reminders))
+        .route("/api/operator-inbox", get(api_operator_inbox))
         .route("/api/stats-hive", get(api_stats_hive))
         .route("/api/container-resources", get(api_container_resources))
         .route("/api/build-logs", get(get_build_logs_all))
@@ -1736,6 +1737,48 @@ async fn api_reminders(State(state): State<AppState>) -> Response {
     }
 }
 
+/// Unread operator-directed messages for the dashboard's Y3R C4LL inbox
+/// (#1469). Returns messages addressed to `"operator"` that haven't been
+/// acked yet (the operator clears them via the existing
+/// `POST /api/agent/operator/mark-all-read`). Newest-first; path-shaped
+/// tokens are validated so the client renders file links like the
+/// terminal does. Shape: `{ "messages": [{ id, from, body, at,
+/// in_reply_to, file_refs }] }`.
+async fn api_operator_inbox(State(state): State<AppState>) -> Response {
+    const INBOX_LIMIT: u64 = 100;
+    match state.coord.broker.unread_for_recipient("operator", INBOX_LIMIT) {
+        Ok(messages) => {
+            let items: Vec<serde_json::Value> = messages
+                .into_iter()
+                .filter_map(|m| match m {
+                    crate::broker::MessageEvent::Sent {
+                        id,
+                        from,
+                        body,
+                        at,
+                        in_reply_to,
+                        ..
+                    } => {
+                        let file_refs = scan_validated_paths(&body);
+                        Some(serde_json::json!({
+                            "id": id,
+                            "from": from,
+                            "body": body,
+                            "at": at,
+                            "in_reply_to": in_reply_to,
+                            "file_refs": file_refs,
+                        }))
+                    }
+                    crate::broker::MessageEvent::Delivered { .. }
+                    | crate::broker::MessageEvent::Ping { .. } => None,
+                })
+                .collect();
+            axum::Json(serde_json::json!({ "messages": items })).into_response()
+        }
+        Err(e) => error_response(&format!("operator-inbox failed: {e:#}")),
+    }
+}
+
 #[derive(Deserialize)]
 struct StatsHiveQuery {
     window: Option<String>,