chore: add Dependabot for automated dependency updates

[ottobot-ai]

Summary

Adds Dependabot configuration to automate dependency updates across all OttoChain repositories.

Changes

• Weekly automated PRs for outdated npm packages (JS repos) and GitHub Actions
• PRs labeled dependencies + automated for easy triage
• chore(deps): commit prefix for consistent changelogs
• Internal @ottochain/* workspace packages excluded from updates
• Max 10 open npm PRs, 5 GitHub Actions PRs at a time

Acceptance Criteria

• Dependabot configured in all 6 active repos
• Weekly PRs for npm (JS repos) and GitHub Actions
• PR title prefix: chore(deps):
• Labels: dependencies, automated auto-applied
• Internal @ottochain/* packages excluded
• Config committed to each repo root (.github/dependabot.yml)

Trello

Card: https://trello.com/c/699678344d3c68750ca7b5c8