M1 #7: Add dependabot.yml for automated dependency updates

[joaquinbejar]

Summary

Add Dependabot configuration for automated weekly dependency update PRs for both Cargo dependencies and GitHub Actions.

Changes

• Add .github/dependabot.yml configuration file
• Configure weekly Cargo dependency updates (Monday, limit 10 PRs, prefix chore(deps))
• Configure weekly GitHub Actions updates (Monday, limit 5 PRs, prefix chore(ci))

Testing

• Unit tests added/updated
• Integration tests added/updated (if applicable)
• Benchmark tests added/updated (if applicable)
• Manual testing performed (make pre-push)

Checklist

• All public items have /// documentation
• No warnings from cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all --check passes
• No .unwrap(), .expect(), or panics in library code
• Minimal dependencies — no unnecessary crates added

Closes #7

[codecov-commenter]

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✅ All modified and coverable lines are covered by tests.

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