[abhinav]

Native stack updates and atomic merges are skipped when
spice.forge.github.apiUrl points to https://api.<tenant>.ghe.com.
The gateway appends /v3 to REST requests for every host except
api.github.com, but GHE.com serves REST at the API host root.
The stack probe interprets the resulting 404 as an unsupported feature,
so callers silently fall back to ordinary PR operations.
GraphQL requests already use the correct /graphql endpoint.

Recognize GHE.com API hosts alongside api.github.com when selecting
the REST base, preserving /api/v3 for Enterprise Server.

Centralize web URL defaults and API endpoint derivation in Deployment:

github.Deployment{URL: o.URL, APIURL: o.APIURL}

The forge forwards its raw URL settings, and the gateway resolves the
protocol endpoints, so deployment URL rules have one owner.
Keep web URL selection independent of API parsing,
which still occurs when opening a repository.

Fixes #1498

[abhinav]

This change is part of the following stack:

• github: Fix native stacks on GHE.com #1499 ◀

_{Change managed by git-spice.}