MCP polish + SSH host manager Connect button

Three small things bundled from PR-3 verification:

1. Split SSH out of mcp.spawn_pane schema. New McpSpawnSpec enum
   (Wsl | Powershell only) used for SpawnPaneArgs, so Claude's
   spawn_pane tool description and JSON schema show only the local
   shells. SSH must go through connect_host. The internal
   pty::SpawnSpec is unchanged — the frontend's manual spawn path
   via XtermPane still supports all three variants. Previously
   spawn_pane(kind=ssh) was a half-broken path that required `host`
   as a separate mandatory field even when hostId was given;
   serde-rejected the natural "spawn to a saved host" call shape.

2. Refresh the MCP server's `with_instructions` text and the
   module-level header comment. Both still claimed "read-only v1"
   long after the v2 write surface landed, which was making Claude
   refuse to attempt tools on first contact ("the server has
   flagged itself as read-only..."). The instructions now describe
   the actual tool set, the SSH-via-connect_host convention, and
   the policy/safeguards gates so Claude doesn't have to infer.

3. Add a "Connect" button to the SSH hosts manager. Previously
   the dialog only had Edit — users (rightly) expected clicking a
   saved host to spawn an SSH pane to it. New onConnect callback
   does the splitLeaf + smart-orient dance and closes the manager.
   Buttons wrapped in a flex container so the row's
   space-between layout doesn't strand the new button mid-row.

src/components/HostManager.css

@@ -92,7 +92,13 @@
   font-size: 11px;
   margin-top: 1px;
 }
-.host-edit-btn {
+.host-actions {
+  display: flex;
+  gap: 6px;
+  flex-shrink: 0;
+}
+.host-edit-btn,
+.host-connect-btn {
   background: #222;
   color: #aac;
   border: 1px solid #2a2a3a;
@@ -106,6 +112,15 @@
   background: #2a2a3a;
   color: #cce;
 }
+.host-connect-btn {
+  background: #1a2a1a;
+  color: #80c080;
+  border-color: #2a4a2a;
+}
+.host-connect-btn:hover {
+  background: #2a4a2a;
+  color: #a0e0a0;
+}
 
 .host-form {
   display: flex;

src/components/HostManager.tsx

@@ -40,6 +40,8 @@ interface HostManagerProps {
   /** Delete the keyring entry for this host id. Called when the user
    *  clicked "Remove password" before Save. */
   onClearPassword: (hostId: string) => void;
+  /** Open a new pane connected to this host (and close the manager). */
+  onConnect: (hostId: string) => void;
   onClose: () => void;
 }
 
@@ -48,6 +50,7 @@ export default function HostManager({
   onSave,
   onSavePassword,
   onClearPassword,
+  onConnect,
   onClose,
 }: HostManagerProps) {
   // Local editable copy. Any save / delete acts on this and pushes the
@@ -377,12 +380,21 @@ export default function HostManager({
                           {h.jumpHost ? ` via ${h.jumpHost}` : ""}
                         </div>
                       </div>
-                      <button
-                        className="host-edit-btn"
-                        onClick={() => startEdit(h.id)}
-                      >
-                        Edit
-                      </button>
+                      <div className="host-actions">
+                        <button
+                          className="host-connect-btn"
+                          onClick={() => onConnect(h.id)}
+                          title={`Open a new pane connected to ${h.label}`}
+                        >
+                          Connect
+                        </button>
+                        <button
+                          className="host-edit-btn"
+                          onClick={() => startEdit(h.id)}
+                        >
+                          Edit
+                        </button>
+                      </div>
                     </div>
                   )}
                 </li>