docs: add a first contribution guide

[afurm]

Draftmora has a concise CONTRIBUTING.md, but it does not yet give first-time contributors a step-by-step path from choosing an issue to opening a small PR.

Suggested scope:

• Add a short "First contribution" section to CONTRIBUTING.md.
• Link the good first issue and help wanted labels.
• Explain the expected local loop: install, run npm run dev, make one focused change, run validation, attach a screenshot for UI changes.
• Remind contributors not to commit .env, SQLite databases, USER.md, MEMORY.md, build output, or personal local state.

Acceptance criteria:

• A newcomer can follow the section without needing maintainer context.
• The guidance stays short and does not duplicate the whole README.
• The PR checklist remains the source of truth for final validation.