docs: Expand CONTRIBUTE.md with a step-by-step contribution workflow

Summary CONTRIBUTE.md lists apps and commands but doesn't tell a first-timer how to contribute (fork, branch, commit format, required checks, changesets, opening a PR). Add that walkthrough.

Where

• CONTRIBUTE.md

Steps

1. Branch: git checkout -b docs/contribute-workflow.
2. Add a ## Submitting a Pull Request section covering, in order:
   • Fork → clone → yarn install.
   • Create a feature branch (feat/..., fix/..., docs/...).
   • Run yarn lint, yarn typecheck, yarn test before pushing.
   • Conventional commits — 2–3 examples (feat(studio): ..., fix(ai-models): ..., docs: ...). The repo uses commitlint.
   • Changesets — user-facing changes need yarn changeset (repo uses the .changeset workflow).
   • Push and open a PR against main.
3. Run yarn lint.
4. Commit docs: add PR workflow to CONTRIBUTE.md and open a PR.

Acceptance criteria

• A newcomer can follow CONTRIBUTE.md end-to-end without prior repo knowledge.
• Conventional-commit and changeset steps are documented.

Good first issue — comment here if you'd like to take it.