M8: GitHub Enterprise Server support

[michaeljolley]

Final milestone of the RayCast parity roadmap. Stacked on top of #294 (M7).

What this adds

Make the GitHub host configurable end-to-end so the extension can target a GitHub Enterprise Server (GHES) instance in addition to github.com.

• Central host resolver (\GitHubHostAddress): normalizes a user-entered host (bare host assumes https, empty means github.com), detects github.com vs GHES, and produces the Octokit REST API base (\https://api.github.com/\ or \https://HOST/api/v3/\) and the GraphQL endpoint (\https://api.github.com/graphql\ or \https://HOST/api/graphql\).
• Shared host logic: \GitHubGraphQLClient\ now derives its endpoint through \GitHubHostAddress, so REST and GraphQL host logic have one source of truth.
• Host-aware OAuth: \OAuthRequest\ accepts an optional host and builds its Octokit client against the enterprise API base; \IDeveloperIdProvider.LoginNewDeveloperIdAsync(host)\ threads the chosen host through sign-in. The parameterless path still defaults to github.com, and restored accounts already carry their own host.
• Sign In form: an optional 'GitHub Enterprise Server host' field (blank keeps github.com); \SignInCommand.Invoke(host)\ passes it through.

Tests

• Host parsing (empty, bare, scheme, unsupported scheme), github.com vs GHES API/GraphQL resolution, and the GraphQL client delegation.
• 221/221 tests pass.

Notes

• OAuth against a live GHES instance can't be exercised in this environment; the host resolution and client construction are unit tested, and the flow reuses the existing github.com OAuth path.
• Per-account host was already supported when restoring saved accounts; this milestone extends it to the initial sign-in.