[codex] Move release platform rules into bazel package (#27321)

## Intent

Keep release-specific Bazel helpers out of the shared Rust crate
definitions and colocate them with Bazel platform configuration.

## Implementation

Moves `multiplatform_binaries` and its platform list from `defs.bzl`
into `bazel/platforms/release_binaries.bzl` and updates the CLI load
site. Behavior is unchanged.

## Validation

- `bazel query //codex-rs/cli:release_binaries`

Stack: 1 of 6.

codex-rs/cli/BUILD.bazel

@@ -1,4 +1,5 @@
-load("//:defs.bzl", "MACOS_WEBRTC_RUSTC_LINK_FLAGS", "codex_rust_crate", "multiplatform_binaries")
+load("//:defs.bzl", "MACOS_WEBRTC_RUSTC_LINK_FLAGS", "codex_rust_crate")
+load("//bazel/platforms:release_binaries.bzl", "multiplatform_binaries")
 
 codex_rust_crate(
     name = "cli",