core's `files` array matches the files the package ships

## Summary

`packages/core/package.json` lists `README.md` in its `files` array, but
`packages/core/README.md` doesn't exist at HEAD.

## Motivation

The entry is inert today: the package is `private: true` at `0.0.0` and nothing
publishes, so `files` is never consulted. It still misleads. In #551 the entry
read as evidence that per-package READMEs were this repo's convention, which is
why that PR gave the new `random` package one before the assumption was
checked. The README came back out, and `random`'s `files` array with it, but
core's claim stands.

Left out of #551 to hold that PR to the extraction it was doing.

## Scope

- Remove `"README.md"` from the `files` array in `packages/core/package.json`.

Removal rather than writing the README: #551 established that a library package
here earns a README only when it carries facts living nowhere else.
`foundry-module` clears that bar (the module ID Foundry registers, the bundle
shape, `module.json` as the load entry, the two how-to guides) and keeps both
its README and its `files` entry. `core` has no equivalent; its usage lives in
the root README, `CONTRIBUTING.md`, and the docstrings.

## Acceptance criteria

- [ ] Every path listed in a `files` array under `packages/*/package.json`
exists at HEAD.
- [ ] Typecheck and lint pass.

## Additional context

Found in #551, which removed the equivalent entry from `packages/random`.