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.

frontend/packages/dashboard/src/index.html

@@ -153,6 +153,16 @@
          can decide without leaving the pane. -->
     <section class="tab-pane" id="tab-pane-call"
              role="tabpanel" aria-labelledby="tab-call">
+      <!-- 1NB0X: unread agent→operator messages (#1469). Fetched on
+           tab activation + cold load, appended live from the broker
+           stream, cleared via "mark all read". Count folds into the
+           Y3R C4LL pill so messages aren't missed. -->
+      <h2>◆ 1NB0X ◆</h2>
+      <div class="divider">══════════════════════════════════════════════════════════════</div>
+      <div id="operator-inbox-section">
+        <p class="meta">loading…</p>
+      </div>
+
       <h2>◆ P3NDING APPR0VALS ◆</h2>
       <div class="divider">══════════════════════════════════════════════════════════════</div>
       <div id="approvals-section">