外部 CLI 新增 exec --terminal：在已打开的终端里执行命令

只允许登录一次的堡垒机（外部连接）上，exec 重新登录会被拒绝，另开通道又会结束会话。
--terminal 把命令打进这台主机在 ShellRS 里已打开的远程终端：先发 Ctrl-C，探测出
交互 shell（bash、dash、busybox ash）后输入命令行，经 OSC 标记取回输出、退出码和
执行上下文（--json 的 context）。不做忙碌检测，只拒绝全屏程序和同一终端已有一次运行。
外部连接上普通 exec 登录失败时，说明里提示改用 --terminal。

shells.yml 在 Docker 里对 bash 4.2–5.2、dash、busybox ash 跑真实终端测试。

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/shells.yml

@@ -0,0 +1,47 @@
+# `shellrs exec --terminal` against the shells it types into: bash 4.2 to
+# 5.2, dash, busybox ash and a bash started as sh, each in a container on a
+# real terminal (`terminal::run::tests::shells`). The Macs this is
+# developed on run it only with Docker Desktop up.
+#
+# A push runs it when the run's code, the test helpers or the dependencies
+# change; run it by hand otherwise (Actions › shells › Run workflow).
+name: shells
+
+on:
+  push:
+    branches: ["**"]
+    paths:
+      - "src/terminal/**"
+      - "src/testing.rs"
+      - "Cargo.toml"
+      - "Cargo.lock"
+      - "rust-toolchain.toml"
+      - ".github/workflows/shells.yml"
+  workflow_dispatch:
+
+jobs:
+  shells:
+    runs-on: ubuntu-22.04
+    timeout-minutes: 90
+    env:
+      CARGO_TERM_COLOR: always
+    steps:
+      - uses: actions/checkout@v4
+      - name: Build dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y cmake clang pkg-config libfontconfig-dev \
+            libfreetype-dev libxkbcommon-dev libxkbcommon-x11-dev libwayland-dev \
+            libx11-xcb-dev libxcb1-dev libvulkan-dev libdbus-1-dev libssl-dev
+      # Pulled first, so that no test waits on a download.
+      - name: Images
+        run: |
+          for image in bash:4.2 bash:4.4 bash:5.0 bash:5.1 bash:5.2 \
+            debian:stable-slim alpine:3 rockylinux:9; do
+            docker pull --quiet "$image"
+          done
+      # rust-toolchain.toml picks the toolchain; this installs it with the
+      # components it lists before the cache key is computed.
+      - run: rustup toolchain install
+      - uses: Swatinem/rust-cache@v2
+      - run: cargo test --lib terminal::run -- --include-ignored