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[chore] Update AGENTS.md

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# Sylva OS — Agent Guide
x86_64 UEFI hobby OS. C17 boot (`.c`) → C++17 kernel (`.cpp`), ELF64 → PE32+ via objcopy.
x86_64 UEFI hobby OS. C17 boot → C++17 kernel, ELF64 → PE32+ via objcopy.
## Build & Run
| Command | What |
|---------|------|
| `make` | Build `build/BOOTX64.EFI` |
| `make run` | QEMU + OVMF, serial port 1 → `serial.log`, serial port 2 → stdio, paused for GDB (`-s -S`) |
| `make clean` | **Always before commit** — gnu-efi build produces untracked files outside `build/` |
| `make` | Build `build/BOOTX64.EFI` + `build/Kernel.elf` |
| `make run` | QEMU + OVMF, serial 1 → `serial.log`, serial 2 → stdio, paused for GDB (`:1234`) |
| `make vdir` | Stage both binaries into `vdir/EFI/BOOT/` for manual boot |
| `make clean` | **Always before commit** — untracked build artifacts leak outside `build/` |
## Architecture
- `boot.c` — UEFI entrypoint (`efi_main` → `kernel_main()`)
- Two-stage boot: `boot.c` (UEFI app) loads `Kernel.elf` from FAT volume, parses ELF PHDRs, jumps to entry
- `kernel/entry.cpp` — `_start` saves `SystemTable`, calls `kernel_main()`
- `kernel/main.cpp` — `extern "C" kernel_main()`: init GOP → serial → PMM → heap → idle (`hlt`)
- `kernel/serial.cpp` — UEFI serial protocol wrapper
- `kernel/serial.cpp` — UEFI serial protocol wrapper (write/read/hex)
- `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 font renderer
- `efi/` — bundled gnu-efi source; `gnu-efi/` is an unregistered submodule reference
- `kernel/memory/heap.cpp` — kmalloc/kfree/kcalloc/krealloc (first-fit w/ coalescing)
- `graphics/` — GOP framebuffer, `fonts/` — Hankaku 8×16 font via `pf_print()`
- `efi/` — bundled gnu-efi sources: `gnuefi/` (crt0, lds, reloc), `lib/`, `inc/`
- Kernel linked at `0x100000` (`kernel/kernel.ld`)
## Quirks
## Conventions
- **`uefi_call_wrapper` required for all UEFI protocol calls** — omitting it causes page-faults (see `kernel/main.cpp:49`)
- `kernel_main` must be `extern "C"` (C→C++ linkage)
- QEMU starts paused — connect GDB to `:1234` to proceed
- Compiler standards: C17 (`boot.c`), C++17 (`kernel/`), `-ffreestanding -fno-stack-protector -fshort-wchar -mno-red-zone`
- Commits: `[type] message` with types `feat`, `fix`, `chore`, `docs` (git log)
- `#define ASM asm volatile` in `include/common.h`
- `BUILD_INFOS.h` — placeholder for CI; `.github/` is empty
- No tests, no CI, no lint/formatter config
- C17 (`boot.c`), C++17 (`kernel/`) — `-ffreestanding -fno-stack-protector -fshort-wchar -mno-red-zone`
- `uefi_call_wrapper` required for all UEFI protocol calls (omission → page-fault)
- `kernel_main` must be `extern "C"` (C→C++ linkage)
- QEMU starts paused: connect GDB to `:1234` to proceed
- No tests, no CI, no lint config