Initial CST-Agent: portable Linux/Termux/Windows coding agent.

Vendored sashidown, OpenAI-compatible env (OPENAI_*), and bootstrap scripts.

Co-authored-by: Cursor <cursoragent@cursor.com>

scripts/fetch_busybox_w32.sh

@@ -0,0 +1,47 @@
+#!/usr/bin/env bash
+# Fetch busybox-w32 into third_party/busybox/busybox.exe (Linux/CI host).
+set -euo pipefail
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+DEST="$ROOT/third_party/busybox"
+mkdir -p "$DEST"
+OUT="$DEST/busybox.exe"
+
+# Override with BUSYBOX_W32_URL if the default 404s / TLS fails.
+URLS=(
+  "${BUSYBOX_W32_URL:-}"
+  "https://frippery.org/files/busybox/busybox-w64-FRP-5007-g82accfc19.exe"
+  "https://frippery.org/files/busybox/busybox.exe"
+)
+
+ok=0
+for URL in "${URLS[@]}"; do
+  [[ -z "$URL" ]] && continue
+  echo "Downloading $URL -> $OUT"
+  if curl -fsSL -L --connect-timeout 20 -o "$OUT" "$URL"; then
+    ok=1
+    break
+  fi
+  echo "failed: $URL" >&2
+done
+
+cat > "$DEST/LICENSE.busybox" << 'EOF'
+BusyBox is licensed under GPLv2. Obtain the corresponding source from the
+upstream busybox-w32 / BusyBox project when redistributing the binary.
+See: https://www.busybox.net/  https://frippery.org/busybox/
+EOF
+
+if [[ "$ok" -ne 1 ]]; then
+  cat >&2 <<'EOF'
+Could not download busybox-w32 automatically.
+Manual steps:
+  1. Visit https://frippery.org/busybox/ or busybox-w32 releases
+  2. Save the 64-bit binary as agent/third_party/busybox/busybox.exe
+  3. Copy it next to cst-agent.exe on Windows releases
+EOF
+  exit 1
+fi
+
+chmod +x "$OUT" || true
+file "$OUT" || true
+ls -la "$OUT"
+echo "OK: place this next to cst-agent.exe on Windows releases."