feat(maestro): copy terminal selection to clipboard

xterm.js has no built-in copy-on-select; mirror onSelectionChange to
navigator.clipboard.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/attach.ts

@@ -450,6 +450,12 @@ export function createAttachConnection(
   const resizeDisp = term.onResize(({ cols, rows }) => {
     sendControl({ type: "Resize", cols, rows });
   });
+  // xterm has no built-in copy-on-select; mirror the selection to the
+  // clipboard ourselves (best-effort — insecure contexts lack the API).
+  const selectionDisp = term.onSelectionChange(() => {
+    const text = term.getSelection();
+    if (text) void navigator.clipboard?.writeText(text).catch(() => {});
+  });
 
   window.addEventListener("resize", onResize);
   const visual = window.visualViewport ?? null;
@@ -505,6 +511,7 @@ export function createAttachConnection(
       visual?.removeEventListener("resize", onResize);
       dataDisp.dispose();
       resizeDisp.dispose();
+      selectionDisp.dispose();
       releaseWebgl();
       try {
         ws.close();