follows up on #12429; rm `--all-features` from flags used with `rust-analyzer` on save to prevent disk space bloat under `target/`.
The existing setting kept destroying my `~/.codex/config.toml` for the reasons mentioned in the comment.
Add recommended extension and configure it to auto format prompt.
Adds a default vscode config with generally applicable settings. Adds more entrypoints to justfile both for environment setup and to help agents better verify changes.