fix: make registry project field required for gitlab/github types

[Dygerydoo]

Summary

• Use Zod discriminated union in RegistryEntrySchema so project is validated at schema level for gitlab/github types, instead of relying on runtime checks
• Export narrowed types (ResolvedGitLabRegistry, ResolvedGitHubRegistry, ResolvedDefaultRegistry) so constructors enforce the constraint at compile time
• Remove runtime null checks in GitLabRegistryClient, GitHubRegistryClient, GitLabRepositoryBrowser, and GitHubRepositoryBrowser
• Update resolver to return properly narrowed types via switch

Test plan

• All 610 existing tests pass
• Added schema tests for rejecting gitlab/github entries without project
• Added schema tests for accepting default entries without project
• Removed obsolete runtime-throw tests (now caught by type system)
• Clean tsc --noEmit

Closes #93