feat: ship native dev tools and target sysroot at 0:/usr

The devkit target in programs/GNUmakefile stages the native binutils
(as, ld, ar, nm, objcopy, objdump, readelf, ranlib, strip, size,
strings, addr2line, c++filt, elfedit as *.elf) into the image at
0:/usr/bin, plus a target-side sysroot: libc headers, montauk/ and
Api/ SDK headers and the freestanding C++ set at 0:/usr/include, and
libc.a + crt1/crti/crtn objects (with empty libm/libstdc++ stand-ins)
at 0:/usr/lib. ld's ldscripts ship under 0:/usr/x86_64-montauk.

The /usr prefix matches the binutils configure prefix, and the kernel
already resolves driveless absolute paths against the cwd drive, so
compiled-in /usr/lib search paths work natively on drive 0.

The shell resolves commands from 0:/usr/bin (after 0:/os) and tab
completion lists it. When toolchain/native/ has not been built, the
devkit step skips cleanly and the image builds as before.

Co-Authored-By: Claude Fable 5 <[email protected]>

toolchain/README.md

@@ -91,6 +91,11 @@ Notes:
   the tools themselves never spawn, and real process plumbing is the
   posix_spawn milestone that needs kernel support (exit codes in
   SYS_WAITPID, fd redirection wiring).
-- The staged tree in toolchain/native/ is not yet integrated into the
-  OS image; that needs a target-side sysroot layout decision (where
-  headers, libc.a and crt objects live on 0:/).
+- The `devkit` target in programs/GNUmakefile ships the staged tools
+  into the OS image at 0:/usr/bin (as.elf, ld.elf, ar.elf, ...) with a
+  target-side sysroot at 0:/usr/include + 0:/usr/lib (libc.a, crt
+  objects, empty libm/libstdc++). The kernel resolves driveless
+  absolute paths ("/usr/lib") against the cwd drive, so the binutils
+  --prefix=/usr layout works natively. The shell searches 0:/usr/bin
+  and tab-completes it. If toolchain/native/ has not been built the
+  devkit step is skipped and the image builds without it.