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.

loop.sh

@@ -111,6 +111,7 @@ while [[ $# -gt 0 ]]; do
 done
 
 export ITERATION=0 MAX_ITERATIONS MODE
+export AGENT_LOOP_ACTIVE=1
 
 # --- Validate ---
 if [[ ! "$MODE" =~ ^(implement|explore|fix)$ ]]; then