chore: add dependabot config for Arbitrum/OffchainLabs npm deps

[douglance]

Summary

• Adds dependabot configuration to automatically create PRs when @arbitrum/* or @offchainlabs/* npm dependencies have updates available
• Scans all directories (including submodules like contracts/ and nitro-testnode/scripts/) for package.json files
• Limited to 5 open PRs at a time, checked daily

Test plan

• Verify .github/dependabot.yml is correct
• After merge, GitHub should start creating dependabot PRs within 24 hours

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 28.63%. Comparing base (b9b3864) to head (d8cd2de).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4457      +/-   ##
==========================================
- Coverage   32.72%   28.63%   -4.09%     
==========================================
  Files         493      493              
  Lines       58290    58290              
==========================================
- Hits        19076    16694    -2382     
- Misses      35868    38492    +2624     
+ Partials     3346     3104     -242