Add rustup for on-demand Rust support, document JS/TS development

Install rustup-init binary from Rust CDN. Users bootstrap Rust with
'rustup-init -y' — persists via devbox-rustup and devbox-cargo volumes.
Add JavaScript/TypeScript development docs (Node.js + npm in base, Bun in OMOS).

Dockerfile

@@ -99,6 +99,14 @@ RUN ARCH=$(case "${TARGETARCH}" in amd64) echo "x86_64" ;; arm64) echo "aarch64"
     rm -rf /tmp/uv-* && \
     uv --version
 
+# rustup — Rust toolchain manager
+# Installs the rustup-init binary only. Users bootstrap Rust with:
+#   rustup-init -y && source ~/.cargo/env
+# Toolchains persist via devbox-rustup and devbox-cargo volumes.
+RUN ARCH=$(case "${TARGETARCH}" in amd64) echo "x86_64" ;; arm64) echo "aarch64" ;; *) echo "x86_64" ;; esac) && \
+    curl -fsSL "https://static.rust-lang.org/rustup/dist/${ARCH}-unknown-linux-gnu/rustup-init" -o /usr/local/bin/rustup-init && \
+    chmod +x /usr/local/bin/rustup-init
+
 # Set locale
 RUN sed -i '/en_US.UTF-8/s/^# //g' /etc/locale.gen && locale-gen
 ENV LANG=en_US.UTF-8