feat(flow): filter the timeline by agent

Adds an agent filter to the FL0W header (#1473): a select listing the
live agents narrows the timeline to messages involving the chosen agent
(matched on `from` OR `to`). Each message row now carries
`data-from`/`data-to`; non-matching rows get `.flow-hidden`. New rows
pick up the active filter at render time; changing the filter re-scans
existing rows. The selection persists in localStorage so a reload or
tab-switch keeps the view. The dropdown is populated from the live
container list (and stays current on container add/remove); a saved
selection survives even if that agent isn't currently listed.

Pure frontend; composes with the sent+delivered collapse (the surviving
collapsed row keeps its `data-from`/`data-to`). Closes #1473.

frontend/packages/dashboard/src/flow.css

@@ -163,3 +163,19 @@ body.flow-shell .flow-main-slim {
    wants `#inbox-section` in the DOM (legacy contract), but we
    surface the messages via the pill/flyout instead. */
 .flow-inbox-headless { display: none !important; }
+
+/* Agent filter (#1473): a select in the FL0W header narrows the
+   timeline to messages involving one agent. Non-matching rows get
+   `.flow-hidden`; the select reuses the journal-unit control look. */
+.flow-filter {
+  font-family: inherit;
+  font-size: 0.85em;
+  background: var(--bg-elev);
+  color: var(--amber);
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  padding: 0.25em 0.5em;
+  margin-left: auto;
+}
+.flow-filter:focus { outline: 1px solid var(--purple); }
+.msgrow.flow-hidden { display: none; }

frontend/packages/dashboard/src/flow.html

@@ -18,6 +18,9 @@
   <header class="logs-header">
     <a class="logs-back" href="/">← dashboard</a>
     <span class="logs-title">FL0W</span>
+    <select id="flow-agent-filter" class="flow-filter" title="filter timeline by agent" aria-label="filter timeline by agent">
+      <option value="">all agents</option>
+    </select>
   </header>
 
   <!-- Operator inbox flyout trigger — count + click → side panel

frontend/packages/dashboard/src/flow.js

@@ -91,8 +91,56 @@ import {
   fetch('/api/state').then((r) => r.ok ? r.json() : null).then((s) => {
     if (!s || !Array.isArray(s.containers)) return;
     for (const c of s.containers) flowContainers.set(c.name, c);
+    populateAgentFilter();
   }).catch(() => { /* graceful: compose just shows `*` and nothing else */ });
 
+  // ─── agent filter (#1473) ───────────────────────────────────────────────
+  // A select in the FL0W header narrows the timeline to messages involving
+  // one agent (matched on `from` OR `to`). Each rendered row carries
+  // `data-from` / `data-to`; non-matching rows get `.flow-hidden`. New rows
+  // pick up the active filter at render time (see renderMsg); changing the
+  // filter re-scans existing rows. Selection persists in localStorage so a
+  // reload / tab-switch keeps the view. Uses `$('msgflow')` for row access
+  // so it works regardless of the message-flow IIFE's local scope.
+  let agentFilter = localStorage.getItem('flow-agent-filter') || '';
+  function rowMatchesFilter(from, to) {
+    return !agentFilter || from === agentFilter || to === agentFilter;
+  }
+  function applyAgentFilter() {
+    const flow = $('msgflow');
+    if (!flow) return;
+    for (const row of flow.children) {
+      const { from, to } = row.dataset;
+      if (from === undefined && to === undefined) continue; // non-message row
+      row.classList.toggle('flow-hidden', !rowMatchesFilter(from, to));
+    }
+  }
+  function populateAgentFilter() {
+    const sel = $('flow-agent-filter');
+    if (!sel) return;
+    const names = [...flowContainers.keys()].sort();
+    sel.replaceChildren();
+    sel.append(el('option', { value: '' }, 'all agents'));
+    for (const n of names) sel.append(el('option', { value: n }, n));
+    // Preserve a saved selection even if that agent isn't in the live
+    // container list (yet / anymore) so the filter doesn't silently reset.
+    if (agentFilter && !names.includes(agentFilter)) {
+      sel.append(el('option', { value: agentFilter }, agentFilter));
+    }
+    sel.value = agentFilter;
+  }
+  {
+    const sel = $('flow-agent-filter');
+    if (sel) {
+      sel.addEventListener('change', () => {
+        agentFilter = sel.value;
+        if (agentFilter) localStorage.setItem('flow-agent-filter', agentFilter);
+        else localStorage.removeItem('flow-agent-filter');
+        applyAgentFilter();
+      });
+    }
+  }
+
   // ─── message flow: shared terminal pane ────────────────────────────────
   // Scroll, pill, backfill + SSE plumbing live in @hive/shared/terminal.
   // What stays here is the broker-message renderer + the page-local
@@ -168,6 +216,11 @@ import {
       } else {
         row.append(ts, ' ', arrow, ' ', from, ' ', sep, ' ', to, ' ', body);
       }
+      // Tag with the participants so the agent filter (#1473) can match
+      // on `from`/`to`, and hide the row up-front if a filter is active.
+      row.dataset.from = ev.from;
+      row.dataset.to = ev.to;
+      if (!rowMatchesFilter(ev.from, ev.to)) row.classList.add('flow-hidden');
       // Register this row so future replies can reference it.
       if (ev.id != null && ev.id > 0) msgRowMap.set(ev.id, row);
       return row;
@@ -244,9 +297,13 @@ import {
         container_state_changed: (ev) => {
           if (ev.container && ev.container.name) {
             flowContainers.set(ev.container.name, ev.container);
+            populateAgentFilter(); // keep the filter dropdown current
           }
         },
-        container_removed: (ev) => { flowContainers.delete(ev.name); },
+        container_removed: (ev) => {
+          flowContainers.delete(ev.name);
+          populateAgentFilter();
+        },
         // Drop every other mutation kind silently — without this
         // they'd fall through to the terminal module's default
         // renderer and clutter the log with JSON dumps. The dashboard