chore: update deps, fix undici CVEs, expand test coverage

Bump @actions/github ^6→^8 and @actions/core ^1→^2 to resolve three
undici security advisories (GHSA-g9mf-h72j-4rw9, GHSA-2mjp-6q6p-2qxm,
GHSA-4992-7rv2-5pvq).

Add a manual Jest mock for @actions/github since its transitive dep
@octokit/core@7 is ESM-only and incompatible with Jest's CJS runner.

Expand tests from 16 to 34: add needPreviousIssue, issueExists, additional
checkInputs edge cases, and run() integration tests covering issue creation,
close-previous, linked-comments, milestone, rotate-assignees, and error
handling. Rebuild dist.

Author: imjohnbo

__mocks__/@actions/github.js

@@ -0,0 +1,31 @@
+'use strict';
+
+const mockOctokit = {
+  rest: {
+    issues: {
+      create: jest.fn(),
+      update: jest.fn(),
+      createComment: jest.fn(),
+      listForRepo: jest.fn()
+    },
+    projects: {
+      listForRepo: jest.fn(),
+      listForUser: jest.fn(),
+      listForOrg: jest.fn(),
+      listColumns: jest.fn(),
+      createCard: jest.fn()
+    }
+  },
+  graphql: jest.fn(),
+  paginate: {
+    iterator: jest.fn()
+  }
+};
+
+module.exports = {
+  context: {
+    repo: { owner: 'owner', repo: 'repo' }
+  },
+  getOctokit: jest.fn().mockReturnValue(mockOctokit),
+  mockOctokit
+};