Add AGENTS.md with Cursor Cloud development environment instructions

AGENTS.md

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+# AGENTS.md
+
+## Cursor Cloud specific instructions
+
+This is a **7-Zip (BetterIcon fork)** codebase — a pure C/C++ native project. No package managers, Docker, or runtime services are used. The toolchain is GCC/G++ and GNU Make.
+
+### Building on Linux
+
+The primary Linux build target is the `Alone2` bundle, which produces the `7zz` standalone console binary supporting all archive formats:
+
+```
+cd CPP/7zip/Bundles/Alone2
+make -j -f makefile.gcc
+```
+
+The output binary is at `_o/7zz` (basic) or `b/g/7zz` (optimized via `cmpl_gcc.mak`).
+
+Other useful bundles:
+- `Bundles/Alone` → `7za` (limited formats: 7z/xz/cab/zip/gzip/bzip2/tar)
+- `Bundles/Alone7z` → `7zr` (7z only, reduced)
+
+See `DOC/readme.txt` for full build options including `cmpl_gcc.mak` (GCC optimized), `cmpl_clang.mak` (Clang), and assembler-accelerated variants.
+
+### Testing
+
+There is no formal test suite. Verify builds by:
+1. Running `7zz` with no arguments (shows version/help).
+2. Compress: `7zz a test.7z <file>`
+3. Extract: `7zz x test.7z -o<output_dir>`
+4. Benchmark: `7zz b`
+
+### Lint / Static Analysis
+
+No linter or static analysis tooling is configured in this repo. The build itself uses `-Werror -Wall -Wextra` (via `cmpl_gcc.mak`), so compiler warnings are treated as errors in the optimized build path.
+
+### Notes
+
+- Build artifacts go into `_o/` (basic makefile.gcc) or `b/g/` (cmpl_gcc.mak). These directories are not tracked by git.
+- Windows GUI components (File Manager, Explorer shell extension) cannot be built on Linux.
+- The `makefile` (no extension) files are for Windows `nmake`; use `makefile.gcc` for Linux/macOS.