feat: implement basic HTTP gateway with Express/Hono

## Description
Create the basic HTTP gateway server that will serve as the foundation for MoltClaw.

## Requirements
- [ ] HTTP server on configurable port
- [ ] /healthz endpoint
- [ ] /api/v1/fleet/register endpoint stub
- [ ] /api/v1/fleet/heartbeat endpoint stub
- [ ] CORS and JSON body parsing middleware

## Skills
TypeScript, Hono or Express