Add hosted API integration test

[gtokman]

Summary

• Adds a hosted API smoke test that runs the docs quickstart with MTA_API_KEY.
• Adds a PR/workflow_dispatch GitHub Actions workflow so the smoke can run in CI when MTA_API_KEY is configured as a repo secret.

Verification

• Fresh project with npm install github:HumanInterfaceDesign/mta-js and the provided key returned arrays for subway arrivals, route-aware nearby stops, and M23 vehicles.
• MTA_API_KEY=*** bun test hosted-api.integration.test.ts
• bun test
• bunx tsc --noEmit

Note: the currently published mta-js@1.0.0 on npm is stale and fails this smoke on stops.near; current main passes. After this merges, publish a new npm version so npm i mta-js consumers get the hosted stops fix.

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[gtokman]

Added the multi-agency roadmap to the README on this PR branch. It calls out future normalized SDK support for MBTA, WMATA, CTA, BART, and adapter-style support for SEPTA, LA Metro, MARTA, and smaller GTFS/realtime-only agencies.