Set up Dependabot for npm and Cargo dependencies

## Summary
Dependencies are not auto-updated. Add a Dependabot config to open weekly PRs for npm and Cargo.

## Files
- new `.github/dependabot.yml`

## Acceptance criteria
- [ ] npm ecosystem for `/sdk`, `/frontend/wallet`, `/packages/agent`, root
- [ ] cargo ecosystem for `/contracts`
- [ ] Weekly schedule
- [ ] Group minor + patch updates