agents: remove send(operator) and the operator inbox; alert on turn failures

Agents reach the operator over matrix now (#5115). A `send` to
`operator` is refused, by the broker and by the MCP allow-list check,
with an error that names the matrix MCP tools. The dashboard's Y3R C4LL
tab goes with it: the operator inbox, `GET /api/operator-inbox`,
`Broker::unread_for_recipient`, and the browser notifications that only
ever fired on operator-bound messages.

The harness no longer posts turn failures and plugin-install failures
to the operator inbox. Both are already logged at warn; a new Grafana
rule, `hh-agent-turn-failed`, fires per agent on the harness's
`turn failed` line.

Dashboard-to-agent messaging (op-send, the agent web UI's message box,
mark-all-read for agent inboxes) is unchanged.

Decisions per mara on #5115: q1 a, q2 a plus a grafana alert, q3 keep
dashboard-to-agent messaging and leave swarm-level code alone.

frontend/packages/dashboard/src/dashboard.html

@@ -35,21 +35,6 @@
           <span class="tab-label">◆ SW4RM ◆</span>
           <span class="tab-count" id="tab-count-swarm" hidden></span>
         </a>
-        <a
-          class="tab"
-          id="tab-call"
-          href="#call"
-          role="tab"
-          aria-controls="tab-pane-call"
-          data-tab="call"
-        >
-          <span class="tab-label">◆ Y3R C4LL ◆</span>
-          <span
-            class="tab-count tab-count-attn"
-            id="tab-count-call"
-            hidden
-          ></span>
-        </a>
         <!-- SYST3M moved to its own standalone page (/core.html, "C0R3"),
            reached from the H0M3 hub — keeps the dashboard tab strip lean.
            The rebuild-queue STATE still lives in tabs.js (it drives the
@@ -132,74 +117,6 @@
           </div>
         </section>
 
-        <!-- Y3R C4LL: things blocked on operator decision — the operator
-         inbox; surfaces full bodies inline so the operator can decide
-         without leaving the pane. -->
-        <section
-          class="tab-pane"
-          id="tab-pane-call"
-          data-tab-pane="call"
-          hidden
-          role="tabpanel"
-          aria-labelledby="tab-call"
-        >
-          <!-- 1NB0X: unread agent→operator messages. 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>
-
-          <!-- Operator-local preferences (browser notifications). Was its own
-           S3TT1NGS page (`/settings.html`); mara moved it in here since a
-           standalone page for one toggle didn't earn its own tile. State
-           lives in the browser's localStorage — NOTIF.bind() (common.js)
-           wires the buttons, same dispatch NOTIF.show() already used for
-           inbox notifications above. -->
-          <h2>◆ PR3F3R3NC3S ◆</h2>
-          <div class="divider">
-            ══════════════════════════════════════════════════════════════
-          </div>
-          <p class="meta">
-            operator-local preferences. these live in the browser's localStorage
-            — they do not sync between devices and do not survive a profile
-            wipe.
-          </p>
-          <h3>◇ browser notifications</h3>
-          <p class="meta">
-            desktop notifications for broker messages addressed to you. requires
-            a secure context (https or localhost). mute silences the
-            notifications without revoking the OS-level permission.
-          </p>
-          <div id="notif-row" class="notif-row">
-            <button
-              type="button"
-              id="notif-enable"
-              class="btn btn-notif"
-              hidden
-            >
-              🔔 enable notifications
-            </button>
-            <button type="button" id="notif-mute" class="btn btn-notif" hidden>
-              🔕 mute
-            </button>
-            <button
-              type="button"
-              id="notif-unmute"
-              class="btn btn-notif"
-              hidden
-            >
-              🔔 unmute
-            </button>
-            <span id="notif-status" class="meta" hidden></span>
-          </div>
-        </section>
-
         <!-- SYST3M pane moved to /core.html (the standalone "C0R3" page):
          meta inputs, rebuild queue, kept state, container load. Reached
          from the H0M3 hub. -->