fix(create): crash when running postinstall task with --no-install (#25616)

## Summary
- Fix segmentation fault in `bun create` when using `--no-install` with
a template that has a `bun-create.postinstall` task starting with "bun "
- The bug was caused by unconditionally slicing `argv[2..]` which
created an empty array when `npm_client` was null
- Added check for `npm_client != null` before slicing

## Reproduction
```bash
# Create template with bun-create.postinstall
mkdir -p ~/.bun-create/test-template
echo '{"name":"test","bun-create":{"postinstall":"bun install"}}' > ~/.bun-create/test-template/package.json

# This would crash before the fix
bun create test-template /tmp/my-app --no-install
```

## Test plan
- [x] Verified the reproduction case crashes before the fix
- [x] Verified the reproduction case works after the fix

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

src/cli/create_command.zig

@@ -80,7 +80,7 @@ fn execTask(allocator: std.mem.Allocator, task_: string, cwd: string, _: string,
         }
     }
 
-    if (strings.startsWith(task, "bun ")) {
+    if (npm_client != null and strings.startsWith(task, "bun ")) {
         argv = argv[2..];
     }
 

test/cli/install/bun-create.test.ts

@@ -144,3 +144,31 @@ for (const repo of ["https://github.com/dylan-conway/create-test", "github.com/d
     expect(await exited).toBe(0);
   }, 20_000);
 }
+
+it("should not crash with --no-install and bun-create.postinstall starting with 'bun '", async () => {
+  const bunCreateDir = join(x_dir, "bun-create");
+  const testTemplate = "postinstall-test";
+
+  await Bun.write(
+    join(bunCreateDir, testTemplate, "package.json"),
+    JSON.stringify({
+      name: "test",
+      "bun-create": {
+        postinstall: "bun install",
+      },
+    }),
+  );
+
+  const { exited, stderr, stdout } = spawn({
+    cmd: [bunExe(), "create", testTemplate, join(x_dir, "dest"), "--no-install"],
+    cwd: x_dir,
+    stdout: "pipe",
+    stdin: "ignore",
+    stderr: "pipe",
+    env: { ...env, BUN_CREATE_DIR: bunCreateDir },
+  });
+
+  const [err, _out, exitCode] = await Promise.all([stderr.text(), stdout.text(), exited]);
+  expect(err).not.toContain("error:");
+  expect(exitCode).toBe(0);
+});