docs: add metering quickstart walkthrough

[alan-provable]

Closes #159

Summary

• Adds an end-to-end README quickstart for the metering and settlement flow.
• Covers service registration, two usage writes, accumulator readback, billing quote, settlement, and final drained readback.
• Notes current local-development API-key behavior and X-Request-Id correlation.

Validation

• npm run build
• npm run lint
• npx prettier --check README.md
• npm test (81 tests passed)
• Ran the documented quickstart against http://127.0.0.1:3001; observed 201, 201, 201, 200, 200, 200, 200 and JSON responses matching the README examples.

[mikewheeleer]

lgtm — merging this 🙌

[alan-provable]

CI failed because package.json and package-lock.json had drifted on the base repo, so I pushed a follow-up commit that syncs the lockfile only. Re-ran validation after the lockfile sync: npm ci --ignore-scripts --no-audit --no-fund, npm run build, npm run lint, npx prettier --check README.md package-lock.json, git diff --check origin/main..HEAD, and npm test. The test suite is passing locally: 81 passed, 0 failed.

[alan-provable]

Thanks for merging this.

If there is a preferred GrantFox reward process for the merged work, or any fixed-scope follow-up you want covered next, I am available. I will wait for maintainer or GrantFox assignment/confirmation before opening more PRs on related issues.