docs: refine CONTRIBUTIONS.md

[vinpogo]

Closes #25

[Copilot]

Pull request overview

Updates the repository’s contributor documentation to reflect current local workflows and project layout, aligning guidance with the monorepo’s tooling (pnpm + demo app) and versioning approach.

Changes:

• Refines the development command list (adds pnpm demo, clarifies pnpm test / pnpm dev).
• Simplifies and rewords the versioning/changesets guidance.
• Updates the project structure section to list packages and the demo app.

Comments suppressed due to low confidence (1)

CONTRIBUTING.md:26

• CI runs formatting in check mode (see mise.toml: oxfmt = "pnpm oxfmt --check"). Since this section is positioned as the pre-commit commands, consider documenting pnpm oxfmt --check (and optionally pnpm oxfmt to apply fixes) so contributors can match CI locally.

# Lint and format code
pnpm oxlint
pnpm oxlint --fix
pnpm oxfmt

[Copilot]

Typo in the Conventional Commits line: "convential" should be "conventional" (and typically phrased as "the Conventional Commits standard").

Suggested change

	> Commit messages should follow [convential commit standard](https://www.conventionalcommits.org/en/v1.0.0/)
	> Commit messages should follow the [Conventional Commits standard](https://www.conventionalcommits.org/en/v1.0.0/)

[Copilot]

The checklist marks adding a changeset as "[optional]", but the Versioning section below says changesets are always required for many code changes. Consider making step 6 conditional (e.g., required when modifying a published package) to avoid contradictory guidance.

Suggested change

	6. [optional] add changeset
	6. If your change affects a published package (see Versioning below), add a changeset: `pnpm changeset`