feat(provider): define and implement GitHub Enterprise support scope

[ClaudiaFang]

Priority

P0 — provider contract clarity

Problem

The product needs an explicit definition of "GitHub Enterprise" support:

• GitHub Enterprise Cloud, or
• self-hosted GitHub Enterprise Server (GHES).

If requests are fixed to api.github.com, GHES is not actually supported even if the UI or documentation implies otherwise.

Plan

• Audit current documentation and UI claims.
• Decide whether GHES is in scope.
• If supported, add configurable web/API base URLs with safe defaults.
• Normalize REST and GraphQL endpoint construction.
• Validate authentication, repository lookup, file APIs, batch commits, and error handling against GHES.
• Define supported GHES version range.
• Add provider capability checks where GitHub.com and GHES differ.
• Update README and settings copy with the exact support boundary.

Acceptance criteria

• Users can clearly tell whether Enterprise Cloud and GHES are supported.
• No GitHub endpoint is silently hardcoded when a custom enterprise host is configured.
• Invalid or unsupported hosts fail with actionable diagnostics.
• Tests cover URL construction and provider initialization.
• Live validation is incorporated into the provider E2E work in chore: add live-credential smoke test across github/gitlab/gitea before merging push/pull/delete changes #57 when credentials/environment are available.