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).

docker-compose.yml

@@ -49,9 +49,16 @@ services:
       # Without this, 'uv python install' must be re-run after container removal.
       - devbox-uv:/home/developer/.local/share/uv
 
+      # Optional: persist Rust toolchains and cargo data
+      # Without this, 'rustup-init' must be re-run after container removal.
+      # - devbox-rustup:/home/developer/.rustup
+      # - devbox-cargo:/home/developer/.cargo
+
       # Optional: AWS credentials/SSO config (not read-only — SSO writes token cache)
       # - ~/.aws:/home/developer/.aws
 
 volumes:
   devbox-data:
   devbox-uv:
+  # devbox-rustup:
+  # devbox-cargo: