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>

.gitignore

@@ -0,0 +1,25 @@
+*.o
+cst-agent
+cst-agent.exe
+tests/test_*
+!tests/test_*.c
+session.jsonl
+.agent/
+fixtures/mini_calc/out/
+fixtures/mini_calc/calc
+fixtures/snake_cpp/snake
+fixtures/snake_cpp/out/
+build/
+build-*/
+dist/
+third_party/busybox/busybox
+third_party/busybox/busybox.exe
+third_party/curl/
+third_party/PDCurses/
+third_party/busybox-w32/
+third_party/busybox-src/
+third_party/prefix-mingw/
+vendor/sashidown/*.o
+vendor/sashidown/src/*.o
+vendor/sashidown/libsashidown.a
+vendor/sashidown/libsashidown-mingw.a