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/Dockerfile

@@ -0,0 +1,43 @@
+# ClaudeOS build environment
+#
+# Provides everything needed to build and test a bare-metal i386 kernel:
+#   - Clang + LLD for cross-compilation to i386-elf
+#   - GRUB 2 tools for generating bootable ISO and floppy images
+#   - QEMU (i386) for running integration tests
+#   - CMake + Ninja for the build system
+#   - xorriso and mtools used internally by grub-mkrescue / grub-mkimage
+
+FROM debian:bookworm-slim
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    # Core build tools
+    clang \
+    lld \
+    cmake \
+    ninja-build \
+    make \
+    git \
+    # GRUB image generation
+    grub-pc-bin \
+    grub-common \
+    xorriso \
+    mtools \
+    # QEMU for running the OS
+    qemu-system-x86 \
+    # Handy utilities
+    file \
+    ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+# Verify the cross-compilation toolchain can produce an i386-elf binary.
+RUN echo 'void _start(void){for(;;)__asm__("hlt");}' > /tmp/test.c \
+    && clang -target i386-elf -m32 -march=i386 -ffreestanding -fno-stack-protector \
+             -nostdlib -fuse-ld=lld -Wl,-e,_start \
+             /tmp/test.c -o /tmp/test.elf \
+    && file /tmp/test.elf | grep -q "ELF 32-bit.*80386" \
+    && echo "Toolchain OK" \
+    && rm /tmp/test.c /tmp/test.elf
+
+WORKDIR /workspaces/claude-os

.devcontainer/devcontainer.json

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+{
+    "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"
+}