feat(files): add folder download as zip

- Add zip folder API endpoint
- Add download button to folder rows
- Enforce max archive size limit
- Document endpoint in OpenAPI

lib/agent/web/index.html

@@ -543,6 +543,20 @@
     tbody tr:hover td { background: rgba(94, 234, 212, 0.04); }
     tbody tr:last-child td { border-bottom: 0; }
     td.name { cursor: pointer; color: var(--accent); font-weight: 500; }
+    td.name.dir { display: flex; align-items: center; gap: 0.5rem; }
+    td.name .dir-label { flex: 1; min-width: 0; }
+    td.name .folder-zip {
+      flex-shrink: 0;
+      padding: 0.15rem 0.45rem;
+      font-size: 0.72rem;
+      font-weight: 500;
+      color: var(--text-muted);
+      background: var(--surface-2);
+      border: 1px solid var(--border);
+      border-radius: 4px;
+      cursor: pointer;
+    }
+    td.name .folder-zip:hover { color: var(--accent-bright); border-color: var(--accent-dim); }
     td.name.file { color: var(--text); font-weight: 400; }
     td.name.file:hover { color: var(--accent-bright); }
     td.actions { white-space: nowrap; text-align: right; }