* docs: align CONTRIBUTING.md with pnpm/Turborepo stack

The initial CONTRIBUTING.md added in #193 documented npm + a `main`
branch + a generic style guide, none of which match the repo's actual
conventions. New contributors following it would fail on the first
`npm install`, then again at the commitlint hook.

This rewrite covers what's actually enforced today:

- pnpm 10.22 + Turborepo, Node engines (≥18 for dev, ≥20.19/≥22.13/≥24
  for release).
- `master` as the base branch.
- Conventional Commits enforced by husky + commitlint, with the
  allowed type list and example subjects.
- antfu ESLint rules that actually flag PRs: `I`-prefixed interfaces,
  `_`-prefixed private members, 4-space indent, complexity caps,
  the `pnpm check-circular` gate.
- Real command surface: `pnpm dev/build/test/coverage/lint/lint:types/
  lint:css/check-circular/e2e/e2e:ui` and the scoped `--filter
  @muyajs/core test:spec[:commonmark|:gfm]` runs.
- Repository layout (packages/core as the only source package today,
  examples/, e2e/, stub packages), with a pointer to CLAUDE.md for
  architecture.
- A "before opening a PR" checklist that mirrors what reviewers ask
  for in practice (lint, types, tests, check-circular, e2e/spec when
  relevant, MIGRATION.md updates for public-API changes).
- Issues-only support routing (Discussions are not enabled on this
  repo).

Refs #193.

* docs: address Copilot review on CONTRIBUTING.md

Four factual corrections flagged in the PR #236 review:

- Code of Conduct: GitHub doesn't have DMs and issues in a public
  repo aren't confidential. Point at GitHub's abuse-reporting flow
  and the `author` email in `package.json` instead.
- `pnpm coverage`: the configured provider in `packages/core/vite.config.ts`
  is `istanbul` (`@vitest/coverage-istanbul`), not V8.
- lint-staged: `.lintstagedrc` only targets `*.css`, not `*.{html,css}`.
- Vitest environment: `vite.config.ts` doesn't set a global
  `environment`. Tests default to Node and opt into happy-dom with
  a `// @vitest-environment happy-dom` directive per file.