fix(docs): update contributor guide for current workspace

[4305Labs]

Motivation

The contributor guide described an outdated three-service development setup,
omitted current workspaces, and included stale environment and database commands.

Changes

• document the pinned Bun and Node.js prerequisites
• distinguish app-local development from the Docker Compose stack
• document the current scoped development scripts
• update the complete apps and packages workspace map
• correct the Drizzle Studio command and verification guidance

Verification

• bun install --frozen-lockfile --ignore-scripts
• bun run --cwd apps/api lint
• bun run --cwd packages/core test
• bun run --cwd packages/core build
• Turbo dry-runs for the documented development, test, and build tasks
• docker compose config --services
• prettier --check CONTRIBUTING.md
• git diff --check

Full workspace test/build execution was not run because email lifecycle
dependencies were intentionally skipped for this documentation-only change.

Related to #8