chore: declare secret target support

plugin.json

@@ -160,5 +160,53 @@
             "arch": "arm64",
             "url": "https://github.com/GoCodeAlone/workflow-plugin-discord/releases/download/v0.1.2/workflow-plugin-discord-darwin-arm64.tar.gz"
         }
-    ]
+    ],
+      "secret_targets": [
+        {
+          "provider": "github",
+          "scopes": [
+            "repo",
+            "env",
+            "org"
+          ],
+          "description": "Stored as GitHub Actions secrets when workflows run on GitHub."
+        },
+        {
+          "provider": "gitlab",
+          "scopes": [
+            "project",
+            "group",
+            "env"
+          ],
+          "description": "Stored as GitLab CI/CD variables when workflows run on GitLab."
+        },
+        {
+          "provider": "vault",
+          "scopes": [
+            "mount"
+          ],
+          "description": "Stored in Vault for application/runtime retrieval."
+        },
+        {
+          "provider": "file",
+          "scopes": [
+            "directory"
+          ],
+          "description": "Stored in a local file-backed secret store."
+        },
+        {
+          "provider": "keychain",
+          "scopes": [
+            "service"
+          ],
+          "description": "Stored in the local OS keychain."
+        },
+        {
+          "provider": "env",
+          "scopes": [
+            "process"
+          ],
+          "description": "Provided by the local process environment."
+        }
+      ]
 }