fix: install Stop hook once at loop startup, not per-iteration

Per-iteration install/remove had a race condition: settings.local.json
was written immediately before CC started, and CC could read the old
file (without the hook) on the first iteration.

Now the hook is installed once when loop.sh starts and removed on exit.
The AGENT_LOOP_ACTIVE env var guard ensures it only fires for CC sessions
spawned by the loop, so keeping it installed the whole time is safe.

.claude-plugin/marketplace.json

@@ -10,7 +10,7 @@
       "name": "agent-loop",
       "source": "./",
       "description": "Autonomous generator-evaluator agent loop for long-running coding tasks. Plan interactively, then execute with full visibility.",
-      "version": "0.10.1",
+      "version": "0.10.2",
       "author": {
         "name": "Sheldon"
       },