fix(github): clarify PostReview doc comment, rename test field to 'want'

Address review feedback from round-3 sonnet review:
- PostReview doc comment now accurately describes vcs.ReviewEvent → GitHub
  wire-format string cast and notes nil-Comments omitempty behavior.
- Rename 'expected' field to 'want' in TestTranslateGitHubReviewState to
  match the project's established naming convention.

github/review_test.go

@@ -362,8 +362,8 @@ func TestDismissReview_401(t *testing.T) {
 
 func TestTranslateGitHubReviewState(t *testing.T) {
 	tests := []struct {
-		input    string
-		expected string
+		input string
+		want  string
 	}{
 		{"APPROVED", "APPROVED"},
 		{"CHANGES_REQUESTED", "REQUEST_CHANGES"},
@@ -374,8 +374,8 @@ func TestTranslateGitHubReviewState(t *testing.T) {
 	}
 	for _, tt := range tests {
 		got := translateGitHubReviewState(tt.input)
-		if got != tt.expected {
-			t.Errorf("translateGitHubReviewState(%q) = %q, want %q", tt.input, got, tt.expected)
+		if got != tt.want {
+			t.Errorf("translateGitHubReviewState(%q) = %q, want %q", tt.input, got, tt.want)
 		}
 	}
 }