A contributing guide, since dev is already live

[WaylandYang]

There was no CONTRIBUTING file, and dev plus its branch protection are already live. Right
now someone arriving with a fix has nothing telling them where the PR goes.

Two files, matching the README convention: CONTRIBUTING.md and CONTRIBUTING.zh-CN.md,
cross-linked.

What they cover, all of it specific to this repository rather than generic etiquette:

• Branch flow. Branch off dev, PR with base dev, maintainers merge dev into main on
  their own schedule. Both branches protected.
• The four checks CI runs, so green locally means green in CI.
• Database-backed tests skip without UTOPIA_DATABASE_URL. The easiest thing to get wrong
  here: a green cargo test --workspace does not mean everything ran, and what those tests
  guard (table aliases inside SQL strings, NULL in comparisons, rows an INNER JOIN drops,
  recursive CTEs under diamond inheritance) is exactly what the compiler cannot see.
• Don't collide migration numbers. Two branches each writing an 0011_ has happened.
• UI strings go in i18n, comments explain why, commit messages are one English sentence.
• DCO sign-off, not a CLA. Contributors keep copyright.

Issue-first is required for anything touching the data model, the ontology contract or a public
API, with an ADR before code. Bug fixes, docs, i18n and tests go straight to a PR.