feat(hook): close mailbox on SessionEnd

New SessionEnd plugin hook runs `claude-mailbox session-end`, which derives the session's auto-name from stdin and asks the daemon to delete the mailbox if it has no pending messages either direction. Renamed mailboxes are preserved (the auto-name no longer exists, so DELETE is a no-op). The daemon-side `SKIP_UPSERT_PATHS` prevents the request from auto-recreating the mailbox. Sweeper remains the safety net for sessions that exit ungracefully.

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

plugin/hooks/hooks.json

@@ -29,6 +29,16 @@
           }
         ]
       }
+    ],
+    "SessionEnd": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "claude-mailbox session-end"
+          }
+        ]
+      }
     ]
   }
 }