docs(formatter): document editor command installation

[Osraka]

Why

The current install steps build dist/server.js, but do not put bend2-fmt-lsp on PATH. The Neovim, Helix, and Emacs examples below them all invoke that command, so following the README alone leaves those editor setups unable to start the server.

What changed

• State the checkout-relative working directory for installation and build.
• Separate running the server directly with node from installing its command for editor use.
• Document npm install -g . after building, with an absolute-path alternative for users who do not want a global install.

Verification

• Reproduced the missing command after npm ci and npm run build in a clean copy.
• Installed into an isolated npm prefix and confirmed the command starts an LSP session.
• npm test in the formatter package: 25 passed.
• git diff --check passed.