chore: add CONTRIBUTING.md

[TaprootFreak]

Summary

Adds a repo-scoped CONTRIBUTING.md with the rules that have been biting contributors and AI assistants:

• Migrations: always generate via `npm run migration`, never hand-write schema migrations. Motivated by PR fix: MROS migration #3613 (hand-written constraint names drifted from TypeORM) and PR ci: block modifications to existing migration files #3614 (migration immutability CI check).
• Exception: data-only migrations (UPDATE/INSERT/DELETE) may be hand-written.
• Canonical JSON-column pattern with typed getter/setter (see `buy-crypto.entity.ts`).
• Create vs Update DTO conventions (`IsOptional` vs `IsOptionalButNotNull`).
• Service update pattern (`Object.assign` + typed setters).
• Module/subdomain structure.
• Git workflow (branch naming, squash-merge, auto release PR).

Filename CONTRIBUTING.md is tool-neutral — doesn't auto-load into Claude Code like CLAUDE.md would, but works as standard contributor documentation and can still be referenced explicitly.

Test plan

• New contributor reads it and doesn't produce the same migration mistakes again