ci: add GitHub Actions workflow + arbitrum blob-fee test

[Pattermesh]

Summary

• Add a GitHub Actions CI workflow (.github/workflows/ci.yml) that runs npm install + npm test on pushes and PRs to main. The repo had test files but no CI.
• Add one meaningful test (tests/chains/arbitrum.test.ts) for the Arbitrum adapter's blob-fee logic: it verifies that computeL2Cost derives the L1 data-fee component from the blob base fee (blobBaseFee * 1600 / 16), adds it on top of the L2 execution fee, scales linearly with the blob fee, and reduces to the bare execution fee at zero blob fee. This adapter was the core piece of blob-fee -> L2 gas-cost logic that had no test coverage (scroll/blast/zksync/mantle/polygonzkevm already had tests).

Test output

 Test Files  7 passed (7)
      Tests  43 passed (43)

(38 -> 43 tests; the new file adds 5.)

Notes

CI runs npm test only. npm run lint currently fails on a pre-existing duplicate shebang in src/cli.ts unrelated to this change, so it is intentionally left out of CI to keep this PR small and CI green.

🤖 Generated with Claude Code

[abhicris]

Welcome to kcolbchain, @Pattermesh — glad you're here. 🌱

Here's what happens from this PR:

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[abhicris]

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