feat(cli): scaffold .github/workflows/ci.yml in junior init (#391)

`junior init` now scaffolds a `.github/workflows/ci.yml` so new Junior
apps ship with CI validation out of the box.

**Scaffolded Workflow**

The generated workflow runs on push to `main` and on pull requests with
steps: checkout, pnpm 10 setup, Node 24, `pnpm install
--frozen-lockfile`, `pnpm check` (validates skills, plugins, and app
structure), and `pnpm build`.

**pnpm versioning**

pnpm version is declared via `pnpm/action-setup`'s `version: 10` input
rather than a `packageManager` field in `package.json`. Scaffolded
projects don't actively maintain a version pin, so `packageManager`
would go stale immediately. Users who want corepack pinning can add it
themselves when ready.

Fixes #390

---------

Co-authored-by: junior[bot] <junior[bot]@sentry.io>
Co-authored-by: Claude (anthropic/claude-opus-4.6) <noreply@anthropic.com>
Co-authored-by: sentry-junior[bot] <264270552+sentry-junior[bot]@users.noreply.github.com>

Author: 3 people

packages/junior/src/cli/init.ts

@@ -56,6 +56,37 @@ function writeVercelJson(targetDir: string): void {
   );
 }
 
+function writeGitHubWorkflow(targetDir: string): void {
+  const workflowDir = path.join(targetDir, ".github", "workflows");
+  fs.mkdirSync(workflowDir, { recursive: true });
+  fs.writeFileSync(
+    path.join(workflowDir, "ci.yml"),
+    `name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      - uses: pnpm/action-setup@v6
+        with:
+          version: 10
+      - uses: actions/setup-node@v6
+        with:
+          node-version: 24
+          cache: pnpm
+      - run: pnpm install --frozen-lockfile
+      - run: pnpm check
+      - run: pnpm build
+`,
+  );
+}
+
 export async function runInit(
   dir: string,
   log: (line: string) => void = console.log,
@@ -158,6 +189,7 @@ SENTRY_ORG_SLUG=
   writeNitroConfig(target);
   writeViteConfig(target);
   writeVercelJson(target);
+  writeGitHubWorkflow(target);
 
   log(`Created ${name} at ${target}`);
   log("");

packages/junior/tests/unit/cli/init-cli.test.ts

@@ -34,6 +34,17 @@ describe("init cli", () => {
     expect(fs.existsSync(path.join(target, "app", "DESCRIPTION.md"))).toBe(
       true,
     );
+    expect(
+      fs.existsSync(path.join(target, ".github", "workflows", "ci.yml")),
+    ).toBe(true);
+
+    const workflow = fs.readFileSync(
+      path.join(target, ".github", "workflows", "ci.yml"),
+      "utf8",
+    );
+    expect(workflow).toContain("pnpm check");
+    expect(workflow).toContain("pnpm build");
+    expect(workflow).toContain("pnpm install --frozen-lockfile");
 
     const vercelConfig = JSON.parse(
       fs.readFileSync(path.join(target, "vercel.json"), "utf8"),