Add devcontainer for i386 kernel development (AI)

Provides a reproducible Debian-based build environment with:
- Clang + LLD for cross-compilation to i386-elf targets
- GRUB 2 tools (grub-pc-bin, xorriso, mtools) for bootable
  ISO and floppy image generation
- QEMU (i386) for integration test execution
- CMake + Ninja for the build system

The Dockerfile includes a self-test that verifies Clang + lld can
produce a valid ELF 32-bit i386 binary before the image is accepted.

.devcontainer/devcontainer.json

@@ -0,0 +1,24 @@
+{
+    "name": "ClaudeOS Dev",
+    "build": {
+        "dockerfile": "Dockerfile",
+        "context": ".."
+    },
+    "customizations": {
+        "vscode": {
+            "extensions": [
+                "ms-vscode.cmake-tools",
+                "ms-vscode.cpptools",
+                "ms-vscode.cpptools-extension-pack"
+            ],
+            "settings": {
+                "cmake.generator": "Ninja",
+                "cmake.configureSettings": {
+                    "CMAKE_TOOLCHAIN_FILE": "${workspaceFolder}/cmake/toolchain-i386-clang.cmake"
+                }
+            }
+        }
+    },
+    "postCreateCommand": "echo 'ClaudeOS devcontainer ready!'",
+    "remoteUser": "root"
+}