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

vcs/interfaces.go

@@ -49,3 +49,12 @@ type Client interface {
 type ReviewerSelfRequester interface {
 	RequestReviewerSelf(ctx context.Context, owner, repo string, number int, user string) error
 }
+
+// ReviewSuperseder is an optional interface implemented by adapters that support
+// marking old reviews as superseded. For Gitea this means editing the review body
+// with a link to the new review and resolving inline comments. For GitHub this
+// means dismissing old reviews.
+// Consumers should use interface assertion: if rs, ok := client.(ReviewSuperseder); ok { ... }
+type ReviewSuperseder interface {
+	SupersedeReviews(ctx context.Context, owner, repo string, prNumber int, oldReviews []Review, newReviewID int64, baseURL, sentinel string) error
+}