[audit/high] Add repository hygiene files (.gitignore, .editorconfig, markdownlint)

[Displacedtex]

Priority: High

Problem

No .gitignore, no .editorconfig, no markdown linting. Even for a README-focused repo this matters: editor droppings (.DS_Store, Thumbs.db) can get committed, and contributor PRs will have inconsistent whitespace/wrap.

Recommendation

• .gitignore — block OS files, editor swap files, node_modules if any tooling is ever added.
• .editorconfig — set markdown line ending + final newline + UTF-8.
• .markdownlint.json (or .markdownlint-cli2.jsonc) — pick a ruleset (e.g. allow long lines in tables).
• .github/workflows/lint.yml — run markdownlint on PRs.

Acceptance criteria

• All four files exist and are consistent.
• PRs with formatting violations fail CI.

Part of the repo audit backlog.