fix: use env var instead of tmux check for Stop hook scoping

The tmux display-message approach had edge cases: it could succeed outside
tmux, fail on first iteration, or behave differently depending on tmux
socket state.

Replace with AGENT_LOOP_ACTIVE env var exported by loop.sh. CC sessions
spawned by the loop inherit it; interactive CC sessions don't. Simple,
no external dependencies, no race conditions.

.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "agent-loop",
-  "version": "0.10.0",
+  "version": "0.10.1",
   "description": "Autonomous generator-evaluator agent loop for long-running coding tasks. Run /agent-loop:run to start.",
   "author": {
     "name": "Sheldon"