fix: keep zsh-fork release assets after removing shell-tool-mcp (#15644)

## Why

`shell-tool-mcp` and the Bash fork are no longer needed, but the patched
zsh fork is still relevant for shell escalation and for the
DotSlash-backed zsh-fork integration tests.

Deleting the old `shell-tool-mcp` workflow also deleted the only
pipeline that rebuilt those patched zsh binaries. This keeps the package
removal, while preserving a small release path that can be reused
whenever `codex-rs/shell-escalation/patches/zsh-exec-wrapper.patch`
changes.

## What changed

- removed the `shell-tool-mcp` workspace package, its npm
packaging/release jobs, the Bash test fixture, and the remaining
Bash-specific compatibility wiring
- deleted the old `.github/workflows/shell-tool-mcp.yml` and
`.github/workflows/shell-tool-mcp-ci.yml` workflows now that their
responsibilities have been replaced or removed
- kept the zsh patch under
`codex-rs/shell-escalation/patches/zsh-exec-wrapper.patch` and updated
the `codex-rs/shell-escalation` docs/code to describe the zsh-based flow
directly
- added `.github/workflows/rust-release-zsh.yml` to build only the three
zsh binaries that `codex-rs/app-server/tests/suite/zsh` needs today:
  - `aarch64-apple-darwin` on `macos-15`
  - `x86_64-unknown-linux-musl` on `ubuntu-24.04`
  - `aarch64-unknown-linux-musl` on `ubuntu-24.04`
- extracted the shared zsh build/smoke-test/stage logic into
`.github/scripts/build-zsh-release-artifact.sh`, made that helper
directly executable, and now invoke it directly from the workflow so the
Linux and macOS jobs only keep the OS-specific setup in YAML
- wired those standalone `codex-zsh-*.tar.gz` assets into
`rust-release.yml` and added `.github/dotslash-zsh-config.json` so
releases also publish a `codex-zsh` DotSlash file
- updated the checked-in `codex-rs/app-server/tests/suite/zsh` fixture
comments to explain that new releases come from the standalone zsh
assets, while the checked-in fixture remains pinned to the latest
historical release until a newer zsh artifact is published
- tightened a couple of follow-on cleanups in
`codex-rs/shell-escalation`: the `ExecParams::command` comment now
describes the shell `-c`/`-lc` string more clearly, and the README now
points at the same `git.code.sf.net` zsh source URL that the workflow
uses

## Testing

- `cargo test -p codex-shell-escalation`
- `just argument-comment-lint`
- `bash -n .github/scripts/build-zsh-release-artifact.sh`
- attempted `cargo test -p codex-core`; unrelated existing failures
remain, but the touched `tools::runtimes::shell::unix_escalation::*`
coverage passed during that run

.github/workflows/rust-release-zsh.yml

@@ -0,0 +1,95 @@
+name: rust-release-zsh
+
+on:
+  workflow_call:
+
+env:
+  ZSH_COMMIT: 77045ef899e53b9598bebc5a41db93a548a40ca6
+  ZSH_PATCH: codex-rs/shell-escalation/patches/zsh-exec-wrapper.patch
+
+jobs:
+  linux:
+    name: Build zsh (Linux) - ${{ matrix.variant }} - ${{ matrix.target }}
+    runs-on: ${{ matrix.runner }}
+    timeout-minutes: 30
+    container:
+      image: ${{ matrix.image }}
+
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - runner: ubuntu-24.04
+            target: x86_64-unknown-linux-musl
+            variant: ubuntu-24.04
+            image: ubuntu:24.04
+            archive_name: codex-zsh-x86_64-unknown-linux-musl.tar.gz
+          - runner: ubuntu-24.04-arm
+            target: aarch64-unknown-linux-musl
+            variant: ubuntu-24.04
+            image: arm64v8/ubuntu:24.04
+            archive_name: codex-zsh-aarch64-unknown-linux-musl.tar.gz
+
+    steps:
+      - name: Install build prerequisites
+        shell: bash
+        run: |
+          set -euo pipefail
+          apt-get update
+          DEBIAN_FRONTEND=noninteractive apt-get install -y \
+            autoconf \
+            bison \
+            build-essential \
+            ca-certificates \
+            gettext \
+            git \
+            libncursesw5-dev
+
+      - uses: actions/checkout@v6
+
+      - name: Build, smoke-test, and stage zsh artifact
+        shell: bash
+        run: |
+          "${GITHUB_WORKSPACE}/.github/scripts/build-zsh-release-artifact.sh" \
+            "dist/zsh/${{ matrix.target }}/${{ matrix.archive_name }}"
+
+      - uses: actions/upload-artifact@v7
+        with:
+          name: codex-zsh-${{ matrix.target }}
+          path: dist/zsh/${{ matrix.target }}/*
+
+  darwin:
+    name: Build zsh (macOS) - ${{ matrix.variant }} - ${{ matrix.target }}
+    runs-on: ${{ matrix.runner }}
+    timeout-minutes: 30
+
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - runner: macos-15-xlarge
+            target: aarch64-apple-darwin
+            variant: macos-15
+            archive_name: codex-zsh-aarch64-apple-darwin.tar.gz
+
+    steps:
+      - name: Install build prerequisites
+        shell: bash
+        run: |
+          set -euo pipefail
+          if ! command -v autoconf >/dev/null 2>&1; then
+            brew install autoconf
+          fi
+
+      - uses: actions/checkout@v6
+
+      - name: Build, smoke-test, and stage zsh artifact
+        shell: bash
+        run: |
+          "${GITHUB_WORKSPACE}/.github/scripts/build-zsh-release-artifact.sh" \
+            "dist/zsh/${{ matrix.target }}/${{ matrix.archive_name }}"
+
+      - uses: actions/upload-artifact@v7
+        with:
+          name: codex-zsh-${{ matrix.target }}
+          path: dist/zsh/${{ matrix.target }}/*