fix(vcs): address Phase 1 review findings

- Rename PullRequest.Head.Sha → SHA (Go acronym convention)
- Add typed ReviewEvent alias with exported constants
- Remove duplicate package doc from types.go (kept in interfaces.go)
- Fix misleading comment in check.go

vcs/check.go

@@ -1,7 +1,7 @@
 //go:build ignore
 
 // This file is excluded from normal builds.
-// Run `go build -v .` inside this file to see the documented gaps between
+// Remove the //go:build ignore tag and run `go build ./vcs/` to see the documented gaps between
 // gitea.Client and vcs.Client that Phase 2 (Gitea adapter) must bridge.
 //
 // Known gaps (as of Phase 1):