ssh 준비 스크립트 추가

Co-authored-by: Copilot <copilot@github.com>

prepare-ssh/action.yml

@@ -0,0 +1,26 @@
+name: SSH 준비
+description: SSH 키를 설정하고 호스트를 known_hosts에 추가하는 스크립트
+
+inputs:
+  ssh-key:
+    required: true
+  host:
+    required: true
+  key-path:
+    required: false
+    default: ~/.ssh/deploy_key
+
+runs:
+  using: composite
+  steps:
+    - shell: bash
+      run: |
+        set -eu
+
+        mkdir -p ~/.ssh
+        chmod 700 ~/.ssh
+
+        printf '%s\n' "${{ inputs.ssh-key }}" > ${{ inputs.key-path }}
+        chmod 600 ${{ inputs.key-path }}
+
+        ssh-keyscan -H "${{ inputs.host }}" >> ~/.ssh/known_hosts