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_test.go

@@ -98,24 +98,6 @@ func TestFormatMarkdown_SpecialChars(t *testing.T) {
 	}
 }
 
-func TestGiteaEvent(t *testing.T) {
-	tests := []struct {
-		verdict  string
-		expected string
-	}{
-		{"APPROVE", "APPROVED"},
-		{"REQUEST_CHANGES", "REQUEST_CHANGES"},
-		{"UNKNOWN", "COMMENT"},
-		{"", "COMMENT"},
-	}
-
-	for _, tc := range tests {
-		got := GiteaEvent(tc.verdict)
-		if got != tc.expected {
-			t.Errorf("GiteaEvent(%q) = %q, want %q", tc.verdict, got, tc.expected)
-		}
-	}
-}
 
 func TestFormatMarkdown_Sentinel(t *testing.T) {
 	result := &ReviewResult{