…builds (#3626)

## Summary

`LocalsKey<T>` (the type returned by `locals.create()`) was branded with
a
module-level `declare const __local: unique symbol`. Each such
declaration
is its own nominal type, and `tshy` emits separate `.d.ts` files for the
ESM and CJS outputs — each gets its own `__local` symbol. Under certain
pnpm hoisting layouts a single TypeScript compilation can resolve
`LocalsKey` from both the ESM source path and the CJS dist path within
the same call site, producing two structurally-incompatible variants of
the same type. TS surfaces this as the misleading error:

```
Argument of type 'LocalsKey<X>' is not assignable to parameter of type
'LocalsKey<X>'. Property '[__local]' is missing in type 'LocalsKey<X>'
but required in type 'BrandLocal<X>'.
```

The error has been hitting CI on PRs opened since the chat.agent stack
landed (e.g. #3625 typecheck job), but doesn't reproduce on developer
machines where the pnpm node_modules layout was built up incrementally.

## Fix

Replace the `unique symbol` brand with an optional phantom field that
carries `T` at the type level:

```ts
// before
declare const __local: unique symbol;
type BrandLocal<T> = { [__local]: T };
export type LocalsKey<T> = BrandLocal<T> & {
  readonly id: string;
  readonly __type: unique symbol;
};

// after
export type LocalsKey<T> = {
  readonly id: string;
  readonly __type: symbol;
  /** Phantom carrier for the value type — never read at runtime. */
  readonly __valueType?: T;
};
```

The ESM and CJS `.d.ts` outputs now produce structurally identical
types,
so cross-output resolution no longer produces a mismatch. `T` is still
carried at the type level via the optional phantom field. The runtime
shape is unchanged — `manager.ts` was already casting via `as unknown`,
which is no longer needed.

## Test plan

- [ ] `pnpm run typecheck --filter @trigger.dev/core --filter
@trigger.dev/sdk`
- [ ] `pnpm run build --filter @trigger.dev/core --filter
@trigger.dev/sdk`
      (clean rebuild) — confirms the ESM and CJS dist `.d.ts` outputs
      no longer carry distinct `unique symbol` declarations
- [ ] `pnpm --filter @trigger.dev/core test test/mockTaskContext.test.ts
--run`
- [ ] `pnpm --filter @trigger.dev/sdk test test/mockChatAgent.test.ts
--run`