Problem: new repos don't inherit standard lint tooling

[zolotokrylin]

Problem

Every Holdex repo is expected to lint markdown on push, but the tooling that enforces it is not standardized at the parent-guidelines level, so each repo reinvents it and they drift.

holdex/developers currently ships .rumdl.toml, .gitignore, a package-lock.json, and .github/workflows/markdown_lint.yml, but has no .githooks/pre-push and no package.json. The actual pre-push hook only lives in holdex/partners:

• https://github.com/holdex/partners/blob/main/.githooks/pre-push

As a result, new repos (including internal holdex/client-* repos) are set up by hand: some lack the pre-push markdown lint, some commit node_modules, and .gitignore coverage varies.

Proposal

Make holdex/developers the source of truth for generic repo hygiene so all repos inherit it:

• Add .githooks/pre-push running rumdl check --fix on changed markdown.
• Add package.json pinning rumdl and wiring the hook via postinstall (git config core.hooksPath .githooks).
• Ensure .gitignore excludes node_modules/, macOS, and Windows junk.
• Document a "Repository setup" step in docs/CONTRIBUTING.md so new repos are scaffolded from this convention rather than ad hoc.

This is the generic half of a split: the client-repo-specific README convention is being handled separately as a rule in holdex/partners.