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>

fixtures/mini_calc/tests/smoke.sh

@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+# Smoke tests for mini_calc. Exit 0 only when add/sub/mul/div are correct.
+set -euo pipefail
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+CALC="$ROOT/calc"
+fail=0
+
+expect() {
+  local op=$1 a=$2 b=$3 want=$4
+  local got
+  got="$("$CALC" "$op" "$a" "$b")"
+  if [[ "$got" != "$want" ]]; then
+    echo "FAIL: $op $a $b → got '$got' want '$want'"
+    fail=1
+  else
+    echo "ok: $op $a $b → $got"
+  fi
+}
+
+expect add 2 3 5
+expect sub 9 4 5
+expect mul 3 4 12
+expect div 8 2 4
+
+if [[ "$fail" -ne 0 ]]; then
+  echo "smoke: FAILED"
+  exit 1
+fi
+echo "smoke: PASSED"
+exit 0