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>

docs/README-windows.md

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+# CST-Agent on Windows
+
+## Release layout
+
+```
+cst-agent.exe
+busybox.exe              # built from busybox-w32
+libwinpthread-1.dll      # MinGW runtime (required)
+LICENSE.busybox
+README-windows.md
+```
+
+Prebuilt tree from Linux cross-compile: `agent/dist/windows-x86_64/`.
+
+## Build (MinGW-w64 cross from Linux)
+
+```bash
+cd agent
+# 1) third-party: curl (Schannel) + PDCurses + busybox-w32
+./scripts/build_third_party_mingw.sh all
+# sources:
+#   https://github.com/rmyorston/busybox-w32.git
+#   https://github.com/curl/curl.git
+#   https://github.com/wmcbrine/PDCurses.git
+# Unix busybox (optional, Linux/Termux bundle):
+#   ./scripts/build_third_party_mingw.sh busybox-unix
+#   https://github.com/mirror/busybox.git
+
+# 2) headless
+cmake -B build-win -DCMAKE_TOOLCHAIN_FILE=cmake/mingw-w64-x86_64.cmake \
+  -DCST_BUILD_TUI=OFF -DCMAKE_PREFIX_PATH=$PWD/third_party/prefix-mingw
+cmake --build build-win
+
+# 3) TUI (PDCurses)
+cmake -B build-win-tui -DCMAKE_TOOLCHAIN_FILE=cmake/mingw-w64-x86_64.cmake \
+  -DCST_BUILD_TUI=ON -DCMAKE_PREFIX_PATH=$PWD/third_party/prefix-mingw \
+  -DPDCurses_ROOT=$PWD/third_party/prefix-mingw
+cmake --build build-win-tui
+```
+
+Copy `build-win-tui/cst-agent.exe`, `third_party/busybox/busybox.exe`, and
+`libwinpthread-1.dll` into one folder.
+
+## Run
+
+```bat
+set CST_API_KEY=...
+cst-agent.exe --headless --workspace . --prompt "echo hi"
+cst-agent.exe --workspace .
+```
+
+Shell: bundled `busybox.exe sh -c ...` unless `CST_SHELL` is set.
+Credentials: `%APPDATA%\cst-agent\credentials` (or console prompt).