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

review/formatter.go

@@ -10,17 +10,6 @@ func FormatMarkdown(result *ReviewResult, reviewerName string) string {
 	return FormatMarkdownWithDisplay(result, reviewerName, reviewerName)
 }
 
-// GiteaEvent converts the verdict to the Gitea API event string.
-func GiteaEvent(verdict string) string {
-	switch verdict {
-	case "APPROVE":
-		return "APPROVED"
-	case "REQUEST_CHANGES":
-		return "REQUEST_CHANGES"
-	default:
-		return "COMMENT"
-	}
-}
 
 // FormatMarkdownWithDisplay formats a ReviewResult with separate display name and sentinel name.
 // Note: displayName is not HTML-escaped as Gitea sanitizes rendered Markdown.