fix: address review feedback - eliminate type assertion via ReviewSuperseder interface

- Introduce vcs.VCSProvider typed constant (replaces plain string provider)
- Introduce vcs.ReviewSuperseder optional interface for supersede logic
- Implement SupersedeReviews on gitea.Adapter (edit + resolve) and
  github.Client (dismiss)
- Remove concrete type assertion client.(*gitea.Adapter) from main
- Remove redundant baseURL fallback for github (NewClient defaults it)
- Condense --gitea-url alias comment block
- Fix fetchPatterns comment (empty paths are skipped, not fetched)
- Add default panic to VCS client init switch

Addresses: #19607, #19608, #19609, #19610, #19621, #19622, #19623

github/conformance_test.go

@@ -9,3 +9,6 @@ import (
 // This verifies github.Client satisfies the full vcs.Client interface
 // (PRReader, FileReader, Reviewer, Identity).
 var _ vcs.Client = (*github.Client)(nil)
+
+// Verify github.Client implements ReviewSuperseder.
+var _ vcs.ReviewSuperseder = (*github.Client)(nil)