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/windows_selftest.md

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+# Windows self-test checklist
+
+Run on a Windows machine (or wine) after MinGW build + busybox.exe beside the binary.
+
+1. `cst-agent.exe --help` exits 0
+2. `set CST_API_KEY=...` then `--headless --workspace . --prompt "run echo hello"` — tool output contains hello
+3. `write_file` under workspace succeeds; path outside workspace rejected
+4. Credentials saved under `%APPDATA%\cst-agent\credentials`
+5. With PDCurses build: launch without `--headless`, confirm TUI draws, `/help` works, Esc cancels
+
+Shell must resolve to bundled busybox unless `CST_SHELL` is set.