build: run buildifier from just fmt (#28125)

## Intent

Keep Bazel and Starlark files consistently formatted without requiring
contributors to install or version buildifier themselves.

## Implementation

- Add a SHA-256-pinned, cross-platform DotSlash manifest for buildifier
v8.5.1.
- Run buildifier from the shared `just fmt` and `just fmt-check` driver,
with Windows-safe explicit DotSlash invocation.
- Provision DotSlash in formatting CI and contributor devcontainers, and
document the source-build prerequisite.
- Apply the initial mechanical buildifier formatting baseline.

justfile

@@ -36,7 +36,7 @@ app-server-test-client *args:
     cargo build -p codex-cli
     cargo run -p codex-app-server-test-client -- --codex-bin ./target/debug/codex {args}
 
-# Format the justfile, Rust, Python SDK code, and Python scripts.
+# Format the justfile, Rust, Bazel/Starlark, Python SDK code, and Python scripts.
 fmt:
     {{ python }} ../scripts/format.py