[BUG] is() should return TypeScript type predicate for narrowing #35

Bug Description

is(err, SomeError) returns a plain boolean, not a TypeScript type predicate. Consumers cannot narrow the error type after the check:

if (is(err, NotFoundError)) {
  // err is still typed as ErrorInstance<unknown> — no narrowing
  console.log(err.fields.slug)  // TS error
}

This forces consumers to cast (err as NotFoundError) or duplicate the narrowing logic. With three try/catch blocks per command and ~7 error types, this churn adds up.

Steps to Reproduce

1. npm install @deessejs/errors
2. Create repro.ts:

import { error, is } from "@deessejs/errors"

const NotFoundError = error({ name: "NotFoundError" })
const OtherError = error({ name: "OtherError" })

const err: unknown = NotFoundError()

if (is(err, NotFoundError)) {
  // Expected: err is now inferred as NotFoundError instance
  // Actual: err is still unknown
  console.log(err)
}

3. Run tsc --noEmit and try to access any field on err inside the if branch.

Expected Behavior

is() should return err is ErrorInstance<TError, ...> (a type predicate), so TypeScript narrows automatically.

Actual Behavior

No narrowing. The if branch treats err as unknown unless the consumer casts.

Environment

• Node.js: 22.x
• pnpm: 11.0.0
• OS: Linux
• @deessejs/errors: 1.1.1

Consumer

deessejs/deessejs monorepo, apps/cli workspace. Would unblock the planned migration of commands/info.ts, commands/list.ts, commands/init.ts to use is(err, CliError) instead of err.name === "CliError".