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>

third_party/busybox/LICENSE.busybox

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+BusyBox is GPLv2. Source: https://github.com/rmyorston/busybox-w32
+Upstream BusyBox: https://github.com/mirror/busybox

third_party/busybox/README.md

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+# BusyBox for CST-Agent
+
+## Windows (`busybox.exe`)
+
+Built from **busybox-w32**:
+https://github.com/rmyorston/busybox-w32.git
+
+```bash
+./scripts/build_third_party_mingw.sh busybox-w32
+# → third_party/busybox/busybox.exe
+```
+
+Ship next to `cst-agent.exe`. License: GPLv2 (`LICENSE.busybox`).
+
+## Linux / Termux (optional side-by-side `busybox`)
+
+Upstream mirror:
+https://github.com/mirror/busybox.git
+
+```bash
+./scripts/build_third_party_mingw.sh busybox-unix
+# → third_party/busybox/busybox
+```
+
+On Unix, `run_shell` prefers `CST_SHELL`, then `<exe_dir>/busybox`, then
+`$PREFIX/bin/sh` (Termux), then `/bin/sh`.