feat: sidebar Write tool, stderr capture, cross-platform URL opener (#584)

Add Write to sidebar allowedTools (both sidebar-agent.ts and server.ts).
Write doesn't expand attack surface beyond what Bash already provides.
Replace empty stderr handler with buffer capture for better error
diagnostics. New bin/gstack-open-url for cross-platform URL opening.

Does NOT include Search Before Building intro flow (deferred).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

bin/gstack-open-url

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+# gstack-open-url — cross-platform URL opener
+#
+# Usage: gstack-open-url <url>
+set -euo pipefail
+
+URL="${1:?Usage: gstack-open-url <url>}"
+
+case "$(uname -s)" in
+  Darwin)  open "$URL" ;;
+  Linux)   xdg-open "$URL" 2>/dev/null || echo "$URL" ;;
+  MINGW*|MSYS*|CYGWIN*) start "$URL" ;;
+  *)       echo "$URL" ;;
+esac