[chore] Create AGENTS.md

AGENTS.md

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+# Sylva OS — Agent Guide
+
+x86_64 UEFI hobby OS kernel. C17 boot → C++17 kernel, ELF64 → PE32+ via objcopy.
+
+## Build & Run
+
+| Command | What |
+|---------|------|
+| `make` | Build `build/BOOTX64.EFI` |
+| `make run` | QEMU + OVMF, serial → `serial.log`, paused for GDB (`-s -S`) |
+| `make clean` | **Always before commit** — gnu-efi produces untracked files |
+
+## Architecture
+
+- `boot.c` — UEFI entrypoint (`efi_main`), calls `kernel_main()`
+- `kernel/main.cpp` — `extern "C" kernel_main()`: init GOP → serial → PMM → heap → idle loop (`hlt`)
+- `kernel/serial.cpp` — UEFI serial protocol wrapper
+- `kernel/memory/pmm.cpp` — bitmap + free-list physical page allocator
+- `kernel/memory/heap.cpp` — kmalloc/kfree/kcalloc/krealloc (first-fit, coalescing)
+- `graphics/` — GOP framebuffer pixel drawing
+- `fonts/` — Hankaku 8×16 pixel font renderer
+- `gnu-efi/` — git submodule (https://github.com/ncroxon/gnu-efi.git)
+
+## Quirks
+
+- **`uefi_call_wrapper` required for all UEFI protocol calls or kernel page-faults**
+- `kernel_main` must be `extern "C"` (C→C++ linkage)
+- QEMU starts paused — connect GDB to :1234 to proceed
+- No tests, no CI, no lint/formatter config
+- C17 (`boot.c`), C++17 (`kernel/`)
+- Standards: `-ffreestanding -fno-stack-protector -fshort-wchar -mno-red-zone`
+- `#define ASM asm volatile` in `include/common.h`
+- `BUILD_INFOS.h` is a placeholder for CI