Add Navigation API support: Directions, Matrix, and Isochrone #75

Request

Add command groups for the Mapbox Navigation APIs — Directions, Matrix, and Isochrone — so an agent can compute routes, travel-time matrices, and reachability from the terminal.

Why

These answer questions a frontier model fundamentally can't, because they depend on the live road network and traffic:

• Directions — route, distance, and traffic-aware travel time between points. "How long to drive from A to B right now, and which way."
• Matrix — travel times/distances across many origins and destinations at once. "Of these 10 candidates, which is fastest to reach from the office." This is the real "travel time between places" primitive.
• Isochrone — the area reachable within N minutes. "What's within a 15-minute walk." Natural pair with search-along-route and site-selection workflows.

Each produces concrete, checkable output an agent can act on, and none of it is memorizable.

Context

The CLI generates commands from the OpenAPI specs vendored in openapi/ and custom-openapi/. None of the Navigation APIs are vendored today, so the commands don't exist. The README notes upcoming releases will extend coverage to most public APIs — this is a request to prioritize the Navigation three.