Ship SP21 collaboration surfaces

[espadonne]

Summary:

• add repository Projects and Wiki storage/domain support with sanitized wiki rendering
• gate private organization repo projects, wikis, and multi-assignee expansion behind Team while preserving public/free behavior
• wire Projects/Wiki repo routes, issue/PR project assignment controls, org pricing rows, docs, and CI lint cleanup

Tests:

• go test ./internal/repos ./internal/issues ./internal/entitlements ./internal/web/...
• make test
• make build
• make ci (from /private/tmp/shithub-ci-sp21 after clearing stale golangci-lint cache)