feat(costs): Wire up gt costs record to Stop hook

Add Claude Code Stop hook that automatically records session costs when
sessions end. The hook calls `gt costs record` which now can derive the
session name from GT_* environment variables (GT_RIG, GT_POLECAT, GT_CREW,
GT_ROLE).

Changes:
- Add deriveSessionName() to infer tmux session name from environment
- Add Stop hook to settings-autonomous.json and settings-interactive.json
- Add unit tests for deriveSessionName function

When a Gas Town session ends, the Stop hook fires and records the session
cost as a bead event. Costs then appear in `gt costs --today` output.

Closes: gt-ntzhc

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.claude/settings.json

@@ -35,6 +35,17 @@
           }
         ]
       }
+    ],
+    "Stop": [
+      {
+        "matcher": "",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "gt costs record"
+          }
+        ]
+      }
     ]
   }
 }