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>

src/tui_stub.c

@@ -0,0 +1,13 @@
+#include "cst_agent.h"
+
+#include <stdio.h>
+
+#ifdef CST_HEADLESS_ONLY
+int cst_tui_run(cst_agent_t *agent) {
+  (void)agent;
+  fprintf(stderr,
+          "TUI was not built into this binary (CST_BUILD_TUI=OFF).\n"
+          "Use --headless --prompt '...'\n");
+  return 1;
+}
+#endif