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.

lib/hooks.sh

@@ -14,11 +14,10 @@
 
 SETTINGS_FILE="${PROJECT_ROOT}/.claude/settings.local.json"
 
-# The hook command checks we're inside the agent-loop tmux session before killing.
-# This prevents the hook from affecting other CC sessions open in the same project.
-# IMPORTANT: Must check $TMUX first — tmux display-message can succeed even outside
-# tmux by falling back to the most recently created session, which would be agent-loop.
-HOOK_COMMAND='[ -n "$TMUX" ] && tmux display-message -p "#{session_name}" 2>/dev/null | grep -q "^agent-loop$" && kill -INT $PPID || true'
+# The hook checks AGENT_LOOP_ACTIVE before killing. This env var is exported by
+# loop.sh and inherited by CC sessions it spawns. Interactive CC sessions in the
+# same project won't have it set, so the hook is a no-op for them.
+HOOK_COMMAND='[ "${AGENT_LOOP_ACTIVE:-}" = "1" ] && kill -INT $PPID || true'
 
 install_hooks() {
     if [ ! -f "$SETTINGS_FILE" ]; then