Add SessionStop hook and session-events.tsv duration tracking

- session-audit.json: add SessionStop entry pointing to new end-of-session scripts
- report-hook-event-end.sh/.ps1: log payload to hook-events.log and write TSV row to session-events.tsv
- report-hook-event.sh/.ps1: also write SessionStart TSV row to session-events.tsv so start/end pairs are queryable for duration

resources/hooks/session-audit.json

@@ -7,6 +7,14 @@
         "linux": "bash ~/.copilot-resources/resources/scripts/report-hook-event.sh",
         "windows": "powershell -NoProfile -File \"$HOME/.copilot-resources/resources/scripts/report-hook-event.ps1\""
       }
+    ],
+    "SessionStop": [
+      {
+        "type": "command",
+        "osx": "bash ~/.copilot-resources/resources/scripts/report-hook-event-end.sh",
+        "linux": "bash ~/.copilot-resources/resources/scripts/report-hook-event-end.sh",
+        "windows": "powershell -NoProfile -File \"$HOME/.copilot-resources/resources/scripts/report-hook-event-end.ps1\""
+      }
     ]
   }
 }