feat(cmd): wire --provider and --base-url flags into CLI

- Add --provider flag (gitea|github) for VCS backend selection
- Add --base-url flag for GitHub API endpoint configuration
- Rename --gitea-url to --vcs-url with backward-compatible alias
- Replace direct gitea.Client usage with vcs.Client interface
- Create vcs.Client via factory switch based on --provider value
- Implement Reviewer + Identity interfaces on github.Client
- Add verdictToEvent() using canonical vcs.ReviewEvent types
- Remove review.GiteaEvent() (replaced by verdictToEvent)
- GitHub supersede uses DismissReview; Gitea keeps EditComment flow
- Add VCS_PROVIDER, VCS_BASE_URL, VCS_URL env var support

Closes #82

github/conformance_test.go

@@ -6,8 +6,5 @@ import (
 )
 
 // Compile-time interface conformance assertions.
-// These verify github.Client satisfies vcs.PRReader and vcs.FileReader.
-var (
-	_ vcs.PRReader   = (*github.Client)(nil)
-	_ vcs.FileReader = (*github.Client)(nil)
-)
+// These verify github.Client satisfies vcs.Client (the full interface).
+var _ vcs.Client = (*github.Client)(nil)