[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.

bazel/platforms/BUILD.bazel

@@ -0,0 +1 @@
+# Release platform rules live in this package.

bazel/platforms/release_binaries.bzl

@@ -0,0 +1,27 @@
+"""Rules for building release binaries across supported target platforms."""
+
+load("@rules_platform//platform_data:defs.bzl", "platform_data")
+
+PLATFORMS = [
+    "linux_arm64_musl",
+    "linux_amd64_musl",
+    "macos_amd64",
+    "macos_arm64",
+    "windows_amd64",
+    "windows_arm64",
+]
+
+def multiplatform_binaries(name, platforms = PLATFORMS):
+    for platform in platforms:
+        platform_data(
+            name = name + "_" + platform,
+            platform = "@llvm//platforms:" + platform,
+            target = name,
+            tags = ["manual"],
+        )
+
+    native.filegroup(
+        name = "release_binaries",
+        srcs = [name + "_" + platform for platform in platforms],
+        tags = ["manual"],
+    )