feat(987): add per-agent three-dot context menu on SW4RM tab

Each agent card in the SW4RM tab now has a ⋮ button that reveals a
dropdown with single-agent lifecycle actions: R3ST4RT, ST0P, ST4RT,
R3BU1LD, DESTR0Y, PURG3. Items are state-aware (disabled when action
doesn't apply). The button is CSS-invisible until the row is hovered
or the menu is open — quiet rows stay clean. DESTR0Y and PURG3 are
hidden for the manager container.

Click-outside and Escape close the menu. A tree-rebuild (on any SSE
state update) also closes it to avoid stale DOM references.

frontend/packages/dashboard/src/tabs.js

@@ -325,6 +325,156 @@ window.marked = marked;
     }
   });
 
+  // ─── per-agent context menu ──────────────────────────────────────────
+  // Three-dot (⋮) button on each agent card for quick single-agent
+  // lifecycle actions without needing to select first. State-aware:
+  // restart/stop disabled when agent is stopped, start disabled when
+  // running, destroy/purge hidden for the manager.
+  // The button is CSS-invisible until the row is hovered (or menu is
+  // open) so it doesn't clutter quiet rows.
+
+  let openAgentMenu = null; // currently open dropdown element, or null
+
+  function closeAllAgentMenus() {
+    if (!openAgentMenu) return;
+    openAgentMenu.hidden = true;
+    const wrap = openAgentMenu.closest('.agent-menu');
+    if (wrap) {
+      wrap.classList.remove('open');
+      const btn = wrap.querySelector('.agent-menu-btn');
+      if (btn) btn.setAttribute('aria-expanded', 'false');
+    }
+    openAgentMenu = null;
+  }
+
+  // Close on any click outside an agent-menu element.
+  document.addEventListener('click', (e) => {
+    if (!e.target.closest('.agent-menu')) closeAllAgentMenus();
+  }, true);
+  // Close on Escape (shares the keydown listener below with selection-clear;
+  // close happens first since no selection change is needed here).
+  document.addEventListener('keydown', (e) => {
+    if (e.key === 'Escape' && openAgentMenu) {
+      closeAllAgentMenus();
+      e.stopPropagation();
+    }
+  }, true);
+
+  // Single-agent POST helper shared by all menu items.
+  async function agentMenuPost(actionPath, name, body) {
+    const url = actionPath + encodeURIComponent(name);
+    try {
+      const resp = await fetch(url, {
+        method: 'POST',
+        headers: body ? { 'Content-Type': 'application/x-www-form-urlencoded' } : {},
+        body: body ? new URLSearchParams(body) : undefined,
+        redirect: 'manual',
+      });
+      const ok = resp.ok || resp.type === 'opaqueredirect'
+        || (resp.status >= 200 && resp.status < 400);
+      if (!ok) {
+        const text = await resp.text().catch(() => '');
+        alert('action failed: ' + resp.status + (text ? '\n\n' + text : ''));
+      }
+    } catch (err) {
+      alert('action failed: ' + err);
+    }
+  }
+
+  function buildAgentMenu(c) {
+    const wrap = el('div', { class: 'agent-menu' });
+    const btn = el('button', {
+      type: 'button',
+      class: 'agent-menu-btn',
+      title: `actions for ${c.name}`,
+      'aria-label': `actions for ${c.name}`,
+      'aria-haspopup': 'menu',
+      'aria-expanded': 'false',
+    }, '⋮');
+
+    const dropdown = el('ul', { class: 'agent-menu-dropdown', hidden: true, role: 'menu' });
+
+    function menuItem(label, opts) {
+      const li = el('li', { role: 'presentation' });
+      const item = el('button', {
+        type: 'button',
+        class: 'agent-menu-item',
+        role: 'menuitem',
+      }, label);
+      if (opts.disabled) {
+        item.disabled = true;
+        if (opts.title) item.title = opts.title;
+      } else {
+        item.addEventListener('click', async () => {
+          closeAllAgentMenus();
+          if (opts.confirm && !confirm(opts.confirm)) return;
+          await agentMenuPost(opts.action, c.name, opts.body || null);
+        });
+      }
+      li.append(item);
+      return li;
+    }
+
+    function menuSep() {
+      return el('li', { class: 'agent-menu-sep', role: 'separator' });
+    }
+
+    dropdown.append(
+      menuItem('↺ R3ST4RT', {
+        action: '/restart/',
+        disabled: !c.running,
+        title: 'agent is stopped',
+        confirm: `restart ${c.name}?`,
+      }),
+      menuItem('■ ST0P', {
+        action: '/kill/',
+        disabled: !c.running,
+        title: 'agent is already stopped',
+        confirm: `stop ${c.name}?`,
+      }),
+      menuItem('▶ ST4RT', {
+        action: '/start/',
+        disabled: c.running,
+        title: 'agent is already running',
+        confirm: `start ${c.name}?`,
+      }),
+      menuItem('↻ R3BU1LD', {
+        action: '/rebuild/',
+        confirm: `rebuild ${c.name}? hot-reloads the container.`,
+      }),
+    );
+
+    if (!c.is_manager) {
+      dropdown.append(
+        menuSep(),
+        menuItem('DESTR0Y', {
+          action: '/destroy/',
+          confirm: `destroy ${c.name}? container removed; state + creds kept.`,
+        }),
+        menuItem('PURG3', {
+          action: '/destroy/',
+          body: { purge: 'on' },
+          confirm: `PURGE ${c.name}? WIPES container, config history, claude creds, and notes. no undo.`,
+        }),
+      );
+    }
+
+    btn.addEventListener('click', (e) => {
+      e.stopPropagation();
+      const wasHidden = dropdown.hidden;
+      closeAllAgentMenus();
+      if (wasHidden) {
+        dropdown.hidden = false;
+        btn.setAttribute('aria-expanded', 'true');
+        wrap.classList.add('open');
+        openAgentMenu = dropdown;
+      }
+    });
+
+    wrap.append(btn, dropdown);
+    return wrap;
+  }
+
   // Re-derive port conflicts from the live containers map. Mirrors the
   // server-side `build_port_conflicts` so the banner reacts to event
   // updates instead of waiting for a /api/state refetch.
@@ -350,6 +500,9 @@ window.marked = marked;
   // See docs/web-ui.md::Topology tree for the rendering contract
   // (forest walk, alphabetical sort, orphan + cycle handling).
   function buildAgentTree(containers) {
+    // Close any open context menu before replacing the DOM tree — the
+    // previous dropdown element would otherwise be a stale reference.
+    closeAllAgentMenus();
     const byName = new Map();
     for (const c of containers) byName.set(c.name, c);
     const children = new Map();      // parent_name → [child_name, …]
@@ -721,7 +874,7 @@ window.marked = marked;
       // here since it opens the side panel rather than a link.
       body.append(drill);
 
-      li.append(icon, body);
+      li.append(icon, body, buildAgentMenu(c));
       ul.append(li);
     }
     root.append(ul);