feat(plugin): add SubagentStop hook to auto-check inbox after subagent runs

Delivers peer messages that arrive during a long-running subagent into the
parent context the moment the Task tool returns, instead of waiting until
the next user prompt. Reuses the existing `claude-mailbox check --hook` so
the mailbox identity stays consistent with UserPromptSubmit.

Plugin 1.2.0 → 1.3.0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plugin/hooks/hooks.json

@@ -19,6 +19,16 @@
           }
         ]
       }
+    ],
+    "SubagentStop": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "claude-mailbox check --hook"
+          }
+        ]
+      }
     ]
   }
 }