Add set_output tool (#205)

mcp/output.ts

@@ -0,0 +1,20 @@
+import { type } from "arktype";
+import type { ToolContext } from "./server.ts";
+import { execute, tool } from "./shared.ts";
+
+export const SetOutputParams = type({
+  value: type.string.describe("the output value to expose as a GitHub Action output"),
+});
+
+export function SetOutputTool(ctx: ToolContext) {
+  return tool({
+    name: "set_output",
+    description:
+      "Set the action output for consumption by subsequent workflow steps. The value will be available as the 'result' output of the action.",
+    parameters: SetOutputParams,
+    execute: execute(async (params) => {
+      ctx.toolState.output = params.value;
+      return { success: true };
+    }),
+  });
+}