feat: send additional_permissions in token exchange request

Parse the ADDITIONAL_PERMISSIONS env var and send it as a JSON body
in the OIDC token exchange request. Permissions are merged on top of
the standard defaults (contents: write, pull_requests: write,
issues: write).

test/parse-permissions.test.ts

@@ -0,0 +1,97 @@
+import { describe, expect, test, beforeEach, afterEach } from "bun:test";
+import { parseAdditionalPermissions } from "../src/github/token";
+
+describe("parseAdditionalPermissions", () => {
+  let originalEnv: string | undefined;
+
+  beforeEach(() => {
+    originalEnv = process.env.ADDITIONAL_PERMISSIONS;
+  });
+
+  afterEach(() => {
+    if (originalEnv === undefined) {
+      delete process.env.ADDITIONAL_PERMISSIONS;
+    } else {
+      process.env.ADDITIONAL_PERMISSIONS = originalEnv;
+    }
+  });
+
+  test("returns undefined when env var is not set", () => {
+    delete process.env.ADDITIONAL_PERMISSIONS;
+    expect(parseAdditionalPermissions()).toBeUndefined();
+  });
+
+  test("returns undefined when env var is empty string", () => {
+    process.env.ADDITIONAL_PERMISSIONS = "";
+    expect(parseAdditionalPermissions()).toBeUndefined();
+  });
+
+  test("returns undefined when env var is only whitespace", () => {
+    process.env.ADDITIONAL_PERMISSIONS = "   \n  \n  ";
+    expect(parseAdditionalPermissions()).toBeUndefined();
+  });
+
+  test("parses single permission and merges with defaults", () => {
+    process.env.ADDITIONAL_PERMISSIONS = "actions: read";
+    expect(parseAdditionalPermissions()).toEqual({
+      contents: "write",
+      pull_requests: "write",
+      issues: "write",
+      actions: "read",
+    });
+  });
+
+  test("parses multiple permissions", () => {
+    process.env.ADDITIONAL_PERMISSIONS = "actions: read\nworkflows: write";
+    expect(parseAdditionalPermissions()).toEqual({
+      contents: "write",
+      pull_requests: "write",
+      issues: "write",
+      actions: "read",
+      workflows: "write",
+    });
+  });
+
+  test("additional permissions can override defaults", () => {
+    process.env.ADDITIONAL_PERMISSIONS = "contents: read";
+    expect(parseAdditionalPermissions()).toEqual({
+      contents: "read",
+      pull_requests: "write",
+      issues: "write",
+    });
+  });
+
+  test("handles extra whitespace around keys and values", () => {
+    process.env.ADDITIONAL_PERMISSIONS = "  actions :  read  ";
+    expect(parseAdditionalPermissions()).toEqual({
+      contents: "write",
+      pull_requests: "write",
+      issues: "write",
+      actions: "read",
+    });
+  });
+
+  test("skips empty lines", () => {
+    process.env.ADDITIONAL_PERMISSIONS =
+      "actions: read\n\n\nworkflows: write\n\n";
+    expect(parseAdditionalPermissions()).toEqual({
+      contents: "write",
+      pull_requests: "write",
+      issues: "write",
+      actions: "read",
+      workflows: "write",
+    });
+  });
+
+  test("skips lines without colons", () => {
+    process.env.ADDITIONAL_PERMISSIONS =
+      "actions: read\ninvalid line\nworkflows: write";
+    expect(parseAdditionalPermissions()).toEqual({
+      contents: "write",
+      pull_requests: "write",
+      issues: "write",
+      actions: "read",
+      workflows: "write",
+    });
+  });
+});