Disable DMG staging for signed macOS promotion (#22900)

## Why
`promote_signed` is now used to finish a release from an externally
signed macOS handoff, but this release path (temporarily) no longer
distributes DMGs. Keeping DMG staging enabled made the handoff
unnecessarily require DMG assets and notarization/stapling validation
even though the promoted release only needs the signed macOS binaries.

## What changed
- Set every `stage-signed-macos` matrix entry to `build_dmg: "false"`,
including the primary macOS bundles.
- Kept the existing DMG staging branch in place behind
`matrix.build_dmg` so it can be re-enabled deliberately later.
- Updated the workflow header comment so the signed handoff contract
asks for signed binaries, not signed DMGs.

The regular signed build path that creates, signs, notarizes, and stages
DMGs is unchanged; this only affects the `promote_signed` handoff path.

.github/workflows/rust-release.yml

@@ -10,7 +10,7 @@
 # archive as a GitHub Release asset, then manually dispatch
 # `release_mode=promote_signed` with `unsigned_run_id` and `signed_macos_asset`.
 # The signed handoff archive should contain target or artifact directories such
-# as `aarch64-apple-darwin/` with signed binaries and signed DMGs.
+# as `aarch64-apple-darwin/` with signed binaries.
 
 name: rust-release
 on:
@@ -644,7 +644,7 @@ jobs:
             bundle: primary
             artifact_name: aarch64-apple-darwin
             binaries: "codex codex-responses-api-proxy"
-            build_dmg: "true"
+            build_dmg: "false"
           - target: aarch64-apple-darwin
             bundle: app-server
             artifact_name: aarch64-apple-darwin-app-server
@@ -654,7 +654,7 @@ jobs:
             bundle: primary
             artifact_name: x86_64-apple-darwin
             binaries: "codex codex-responses-api-proxy"
-            build_dmg: "true"
+            build_dmg: "false"
           - target: x86_64-apple-darwin
             bundle: app-server
             artifact_name: x86_64-apple-darwin-app-server
@@ -770,6 +770,9 @@ jobs:
             codesign --verify --strict --verbose=2 "$release_path"
           done
 
+          # DMG staging is disabled for signed promotion because we no longer
+          # distribute DMGs from this release path. Keep the branch here so the
+          # handoff can opt back in by flipping matrix.build_dmg if needed.
           if [[ "${{ matrix.build_dmg }}" == "true" ]]; then
             dmg_name="codex-${target}.dmg"
             dmg_source="${source_dir}/${dmg_name}"