tools: warn that the Stop hook kills the emulator at every turn boundary

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PMRJjbxLqZtsb5Vb7KunPE

tools/re-capture/README.md

@@ -60,3 +60,21 @@ kill -9 <explicit pid>                      # look it up first, then kill by pid
 
 The `snapshot-bash` filter is the reliable tell: the wrapper's command line
 always contains the shell-snapshot path.
+
+## 🔴 The emulator does not survive the end of an agent turn
+
+`/work/.claude/settings.json` defines a **`Stop` hook** that `kill -9`s every
+`xenia_canary` when a turn ends, printing *"Stop hook killed N stale xenia
+process(es)"*.
+
+So:
+
+* **never** launch a run intending to read it in a later turn — it will be dead;
+* an experiment has to produce its evidence **within the turn that starts it**;
+* prefer measurements that land in the first minutes of flight. `REMAINING OB`
+  steps `4 → 8 → 12` inside the first few minutes, which is why a two-pass
+  differential works in one turn while a 25-minute freeze watch does not.
+
+This cost three iterations of investigating a "mysterious external SIGKILL",
+complete with cgroup and host memory forensics, before the hook was found. When
+a process dies at a session boundary, **check the harness first**.