feat: V5a show relationship/index <name> detail views

Fold the singular per-item forms into Command::ShowList { kind,
name: Option<String> } (name: Some = one item). Two grammar
branches reuse the relationship/index completion sources; worker
do_show_one renders a labelled detail block or a friendly
"No ... named X." line, reusing the V5 render path. Help +
parse-usage entries, two ADR-0042 near-miss rows, 5 integration
tests. Mark V5a [x] — V5's [<name>] clause now complete.

src/input_render.rs

@@ -2061,6 +2061,8 @@ mod tests {
                 "tables".to_string(),
                 "relationships".to_string(),
                 "indexes".to_string(),
+                "relationship".to_string(),
+                "index".to_string(),
             ],
         );
     }