[Hook architecture](14) Put the cat-mode-default hook onto the shared hook code

engine/hooks/cat-mode-default/README.md

@@ -60,13 +60,22 @@ It stays silent when the flag is off or when the prompt already mentions
 cat-mode anywhere (a parent that told the subagent to read it gets no
 second copy). Same flag resolution as the prompt hook.
 
+The shared registry keeps this hook in `warn` mode. Set
+`CATSTACK_HOOK_MODE_CAT_MODE_DEFAULT=off|warn|stop` for a machine-local
+override; `stop` turns the `PreToolUse` (Agent) companion into a real block
+(exit 2) instead of rewriting the subagent's prompt. Detection or metrics
+failures allow the harness action.
+
 ## Files
 
-- `detect.py`: flag resolution, typed `/cat-mode` detection, context text.
-- `claude_prompt_submit.py`: the Claude entrypoint; fail-open, never denies.
-- `claude.prompt.hook.json`: settings fragment `install_claude_hook.py` merges.
-- `claude_pretooluse_agent.py` + `claude.agent.hook.json`: the `PreToolUse`
-  (`Agent`) companion that carries the default into subagent prompts.
+- `detect.py`: flag resolution, typed `/cat-mode` detection, context text,
+  and `detect(event)`, the SDK entrypoint returning `Finding` objects.
+- `claude_prompt_submit.py` / `claude_pretooluse_agent.py`: thin calls into
+  `engine/hooks/_sdk/runtime.py`. The agent finding carries
+  `output={"updatedInput": ...}`, which the shared renderer emits as
+  `updatedInput` rather than its generic `additionalContext`.
+- `claude.prompt.hook.json` / `claude.agent.hook.json`: settings fragments
+  `install_claude_hook.py` merges.
 - `tests/fixtures/*.json`: one scenario each (fires / silent) with the
   environment, optional `.env` content, and payload.
 - `tests/fixtures/agent_*.json`: the same for the Agent-tool companion.

engine/hooks/cat-mode-default/tests/test_agent_hook.py

@@ -13,6 +13,7 @@
 import json
 import os
 import sys
+import tempfile
 import unittest
 from contextlib import redirect_stderr, redirect_stdout
 from unittest.mock import patch
@@ -150,6 +151,19 @@ def test_unrelated_prompt_does_not_count(self) -> None:
 
 
 class FailOpenCase(unittest.TestCase):
+    def setUp(self) -> None:
+        self.tmp = tempfile.TemporaryDirectory()
+        self.metrics_env = patch.dict(
+            os.environ,
+            {"CATSTACK_HOOK_METRICS_DIR": self.tmp.name},
+            clear=False,
+        )
+        self.metrics_env.start()
+
+    def tearDown(self) -> None:
+        self.metrics_env.stop()
+        self.tmp.cleanup()
+
     def test_malformed_stdin_prints_nothing(self) -> None:
         out = io.StringIO()
         with patch.object(sys, "stdin", io.StringIO("not json")):

engine/hooks/cat-mode-default/tests/test_hooks.py

@@ -260,6 +260,19 @@ def test_flags_missing_install_instead_of_dead_path(self) -> None:
 
 
 class FailOpenCase(unittest.TestCase):
+    def setUp(self) -> None:
+        self.tmp = tempfile.TemporaryDirectory()
+        self.metrics_env = patch.dict(
+            os.environ,
+            {"CATSTACK_HOOK_METRICS_DIR": self.tmp.name},
+            clear=False,
+        )
+        self.metrics_env.start()
+
+    def tearDown(self) -> None:
+        self.metrics_env.stop()
+        self.tmp.cleanup()
+
     def test_malformed_stdin_prints_nothing(self) -> None:
         out = io.StringIO()
         with patch.object(sys, "stdin", io.StringIO("not json")):