Create directory structure (AI)

Set up the project directory structure as specified in the README:
- apps/: for application source code (e.g. ls, sh)
- docs/: for subsystem documentation
- libs/: for shared libraries used by apps and/or the kernel
- src/: for kernel source code
- src/it/: for integration tests
- vendor/: for 3rd-party source code (AI will not modify this)

The build/ directory is excluded via .gitignore as it contains only
build artifacts. ISO and floppy image files are also gitignored.

.gitignore

@@ -0,0 +1,3 @@
+build/
+*.iso
+*.img