feat: revive the editor studio

The writing-style studio is parked in `scratch/editor-studio` (gitignored, `node_modules`/`dist` pruned). It is a local single-page app for testing a style skill: drop in text, apply the skill, see the output with reasoning and footnoted annotations.

It predates this repo, so it still points at the old private `writing-style` skill that no longer exists.

To pick it back up:

- Point the default `skillUrl` in `DEFAULT_CONFIG` at this repo instead of the dead `dwmkerr-claude-toolkit` one.
- Decide whether it lives here as a real directory or stays parked in scratch.
- Its own backlog is in `scratch/editor-studio/todo.md`: demo mode via `dwmkerr/mock-llm`, streaming output, word-level diff, prompt presets, shareable URL config.

Start in scratch/editor-studio and inspect DEFAULT_CONFIG to find the obsolete skillUrl, then read scratch/editor-studio/todo.md for the existing backlog. Decide from the repository structure whether the studio should remain in scratch or become a real directory. Done means the app points at this repository and the chosen location and scope are documented.

Written by the indexing model from the issue text.