Craft workspace-level github override needs partial schema, not GitHubGlobalConfigSchema

Trap: reusing GitHubGlobalConfigSchema (requires owner+repo) for a workspace's per-workspace github override field looks natural — it's the same shape as top-level github. But a workspace override must allow specifying ONLY projectPath while inheriting owner/repo from the base config; requiring owner/repo again broke validateConfiguration with 'workspaces.<name>.github.owner: Required' for every workspace test config. Fix: give the workspace-level github field its own partial schema (all fields .optional()), while the top-level github schema stays required (non-passthrough per [[019f85aa-3d99-71cd-95ec-afc4cb7f065f]]). Found in getsentry/craft PR #848 (feat/workspaces-schema).