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.

.devcontainer/devcontainer.json

@@ -10,6 +10,10 @@
      case your host is x86 (or vice-versa). */
   "runArgs": ["--platform=linux/arm64"],
 
+  "features": {
+    "ghcr.io/facebook/devcontainers/features/dotslash:latest": {}
+  },
+
   "containerEnv": {
     "RUST_BACKTRACE": "1",
     "CARGO_TARGET_DIR": "${containerWorkspaceFolder}/codex-rs/target-arm64"

.devcontainer/devcontainer.secure.json

@@ -23,6 +23,9 @@
     "--cap-add=NET_RAW"
   ],
   "init": true,
+  "features": {
+    "ghcr.io/facebook/devcontainers/features/dotslash:latest": {}
+  },
   "updateRemoteUserUID": true,
   "remoteUser": "vscode",
   "workspaceMount": "source=${localWorkspaceFolder},target=/workspace,type=bind,consistency=delegated",