fix: prevent zombie process accumulation by verifying subprocess exit (#1168) (#1175)

Two changes fix the observer process resource leak:

1. Add ensureProcessExit to generator finally blocks in SessionRoutes and
   worker-service, matching the pattern already working in SDKAgent.

2. Add stale session reaper (every 2m) that removes sessions with no active
   generator and no pending work after 15m idle. This unblocks the orphan
   reaper which previously skipped processes for "active" sessions.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

plugin/hooks/hooks.json

@@ -8,7 +8,7 @@
           {
             "type": "command",
             "command": "${CLAUDE_PLUGIN_ROOT}/scripts/setup.sh",
-            "timeout": 120
+            "timeout": 300
           }
         ]
       }