fix(ci): correct bidirectional relationship check target (#515)

Check 9 tested `target in npc_rels` which missed NPCs with no
relationship entries. Changed to `target in self.npcs` so the
advisory warning fires correctly for all NPCs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tooling/validate-content

@@ -472,8 +472,8 @@ class ContentIndex:
                 if pair in checked:
                     continue
                 checked.add(pair)
-                if target in npc_rels and cid not in npc_rels.get(target, set()):
-                    print(f"XREF WARNING: {cid} has relationship to {target} but no reciprocal found")
+                if target in self.npcs and cid not in npc_rels.get(target, set()):
+                    print(f"XREF WARNING: {cid} has relationship to {target} but {target} has no reciprocal entry")
                     warnings += 1
         return warnings