[Bug] isCancel no longer narrows prompt results after 1.5.0

[dreyfus92]

Environment

• OS: macOS
• Node Version: v24.20.0
• Package: @clack/core, @clack/prompts
• Package Version: core 1.5.0, prompts 1.8.0

Describe the bug

#592 changed isCancel from value is symbol to value is typeof CANCEL_SYMBOL, but every prompt still returns Promise<T | symbol> (core Prompt.prompt() and the 11 as Promise<... | symbol> casts in prompts). typescript can't subtract a unique symbol from symbol, so after the guard the result still has symbol in its type and any downstream use errors. this breaks code that type-checked fine on 1.7.0.

To Reproduce

import { path, isCancel } from '@clack/prompts';

const p = await path({ message: 'pick a file' });
if (isCancel(p)) process.exit(0);
console.log(`selected: ${p}`);
// TS2731: Implicit conversion of a 'symbol' to a 'string' will fail at runtime.

same snippet passes on prompts 1.7.0 / core 1.4.3. assigning the result to a string fails with TS2322 instead.

Expected behavior

isCancel narrows the cancel case away like it did before. #554 asked for the specific cancel type on prompt return types, and #592 only did the guard half, which is what left the two out of sync.