agent: 🔓 logout in overflow menu + /logout slash command (#576 frontend half)

Pairs with damocles PR #582 (POST /api/logout backend on the
per-agent web UI). Three additions to the agent's app.js:

- New `postLogout` helper next to postCancelTurn / postCompact /
  postNewSession. Same postSimple shape.
- New entry in SLASH_COMMANDS so /help lists /logout.
- New /logout case in handleSlashCommand with window.confirm.
- New '🔓 logout' item in populateOverflowMenu's overflow popover,
  mirroring the new-session item's pattern (confirm before POST,
  disable button while in-flight, closeOverflowMenu before fire).

Confirm dialog spells out the consequences (SIGINT, creds wiped,
park in needs-login) so the operator doesn't accidentally drop a
production session. Tooltip on the menu item links the action back
to the credentials directory + post-logout state.

Wire-level: POST /api/logout, no body. Backend returns 200 with a
text body describing the wipe outcome — postSimple ignores it
(success → no terminal note; failure → red turn-end-fail row).

frontend/packages/agent/src/app.js

@@ -218,6 +218,34 @@ window.marked = marked;
     });
     menu.append(newSessBtn);
 
+    // 🔓 logout (#576) — SIGINTs claude, wipes the credentials dir,
+    // flips the harness LoginState to NeedsLogin. The turn loop's
+    // next iteration parks in wait_for_login; a fresh `claude auth
+    // login` from the dashboard re-arms it (#542 mtime resumption).
+    // Destructive — the operator has to re-paste OAuth creds on the
+    // login screen after — so we confirm with a clear-eyed prompt.
+    const logoutBtn = el('button', {
+      type: 'button',
+      class: 'overflow-item overflow-item-logout',
+      role: 'menuitem',
+      id: 'logout-btn',
+      title: 'wipe ~/.claude/ credentials + park in needs-login until a fresh `claude auth login` runs',
+    },
+      el('span', { class: 'overflow-item-icon', 'aria-hidden': 'true' }, '🔓'),
+      'logout',
+    );
+    logoutBtn.addEventListener('click', () => {
+      if (!window.confirm(
+        `log ${label} out? this SIGINTs any running claude turn, wipes the credentials directory, ` +
+        `and parks the agent in 'needs login' until you paste a fresh OAuth code from the login screen. ` +
+        `prior --continue context is not affected (only the OAuth creds).`
+      )) return;
+      logoutBtn.disabled = true;
+      closeOverflowMenu();
+      postLogout().finally(() => { logoutBtn.disabled = false; });
+    });
+    menu.append(logoutBtn);
+
     overflowMenuPopulated = true;
   }
 
@@ -363,6 +391,7 @@ window.marked = marked;
     { name: '/compact',     desc: 'compact the persistent claude session' },
     { name: '/model',       desc: '/model <name>  — switch claude model for future turns' },
     { name: '/new-session', desc: 'next turn runs without --continue (fresh claude session)' },
+    { name: '/logout',      desc: 'wipe ~/.claude/ credentials + park in needs-login' },
   ];
 
   async function postModel(name) {
@@ -402,6 +431,7 @@ window.marked = marked;
   const postCancelTurn = () => postSimple('/api/cancel',  '/cancel');
   const postCompact    = () => postSimple('/api/compact', '/compact');
   const postNewSession = () => postSimple('/api/new-session', '/new-session');
+  const postLogout     = () => postSimple('/api/logout', '/logout');
 
   function handleSlashCommand(line) {
     if (!termAPI) return false;
@@ -430,6 +460,15 @@ window.marked = marked;
           postNewSession();
         }
         return true;
+      case '/logout':
+        if (window.confirm(
+          `log out? this SIGINTs any running claude turn, wipes the credentials directory, ` +
+          `and parks the agent in 'needs login' until you paste a fresh OAuth code from the login screen. ` +
+          `prior --continue context is not affected (only the OAuth creds).`
+        )) {
+          postLogout();
+        }
+        return true;
       case '/model': {
         const parts = trimmed.split(/\s+/);
         if (parts.length < 2 || !parts[1]) {