Wrap remaining billing endpoints (1 missing of 7)

[ldraney]

Parent Issue

#86 — Expand Octokit coverage

Category: billing

File: src/tools/categories/billing.ts
Current: 6/7

Already Wrapped

getGithubActionsBillingOrg, getGithubActionsBillingUser, getGithubPackagesBillingOrg, getGithubPackagesBillingUser, getSharedStorageBillingOrg, getSharedStorageBillingUser

Methods to Add (1)

Method	Route
getGithubBillingUsageReportOrg	GET /orgs/{org}/settings/billing/usage

Pattern

See src/tools/categories/git.ts. Tool name: github_billing_<snake_case_method>.

Acceptance Criteria

• Method wrapped
• npm run build passes
• TOOLS.md updated