docs(benchmarks): stop native_hook_spawn claiming to be the per-chunk path

The journey's comment, docstring, README row and CLI description all say
it spawns "the per-chunk MessageDisplay hook exactly as Claude Code
does". That stopped being true when MessageDisplay moved to a /bin/sh
appender and evaluate-policy moved to a curl against the runner's relay.
Both are pinned by tests — test_message_display_shell_command_round_trips
asserts "python" is absent from the installed command — so the number the
journey reports (~40ms here) is not on any per-chunk or per-tool-call
path.

Left as it was, the number reads as ~40ms of blocked TUI per streamed
chunk, which would make it the largest single cost in the system and the
obvious thing to go fix. It isn't, and I went and measured a replacement
for an optimization the repo already has.

Say what it measures instead: the lifetime of a hook that is Python,
which is what the per-turn hooks (SessionStart / Stop / UserPromptSubmit
/ PreCompact / Task*), the PostToolUse TodoWrite+TaskUpdate matchers, and
the policy hook's pre-relay fallback still pay — and which is the
standing argument for keeping the hot paths off the interpreter. Naming
the tests that pin it points the next reader at the evidence rather than
at a stale comment.

No behaviour change; comments, docstring, description and README only.

Co-authored-by: Isaac <no-reply@databricks.com>
Signed-off-by: harry-yao_data <harry.yao@databricks.com>

dev/benchmarks/omnigent/README.md

@@ -72,11 +72,22 @@ they still work with no runner or LLM.
 
 | Journey | Operation timed |
 | --- | --- |
-| `native_hook_spawn` | Spawn the per-chunk `MessageDisplay` hook exactly as Claude Code does — isolated interpreter, module entrypoint, JSON payload on stdin |
+| `native_hook_spawn` | Spawn one **Python** command hook — isolated interpreter, module entrypoint, JSON payload on stdin — and time its whole lifetime |
 
 Claude Code **blocks its TUI** on command hooks, so one hook subprocess's
-lifetime is user-visible streaming latency, and the same interpreter+import
-cost fronts every statusline refresh and per-tool-call policy hook. The
+lifetime is user-visible latency. Read this number as *"what a hook costs if it
+is Python"*.
+
+It is **not** the per-chunk streaming cost, and treating it as one leads
+straight to wasted work. The hooks that fire per chunk (`MessageDisplay`) and
+per tool call (`evaluate-policy`) were deliberately moved off the interpreter —
+a `/bin/sh` appender and a `curl` to the runner's relay — and
+`test_message_display_shell_command_round_trips` pins that by asserting
+`"python"` is absent from the installed command. What still pays this number is
+the per-turn set (`SessionStart` / `Stop` / `UserPromptSubmit` / `PreCompact` /
+`Task*`), the `PostToolUse` `TodoWrite`+`TaskUpdate` matchers, and the policy
+hook's Python fallback before the relay is up. So the journey's real job is to
+keep the argument for staying off the interpreter measurable. The
 journey needs no server or runner; registering it here rides hook spawn cost
 on the same nightly/release regression comparison as everything else
 (`omnigent/__init__` re-exports lazily so this stays ~interpreter-sized). The

dev/benchmarks/omnigent/journeys.py

@@ -881,12 +881,21 @@ async def _measure_cli_startup(env: BenchEnvironment, _ctx: JourneyContext) -> N
 # ── native hook spawn (no server involved) ───────────────────
 
 # Claude Code blocks its TUI on command hooks, so one hook subprocess's whole
-# lifetime is user-visible latency: the MessageDisplay hook runs once per
-# streamed text chunk, and the same interpreter+import cost fronts every
-# statusline refresh and per-tool-call policy hook. Spawn the per-chunk hook
-# exactly as Claude Code does — isolated interpreter, module entrypoint, JSON
-# payload on stdin — and time the full process lifetime. The import-graph side
-# of this guarantee is pinned by tests/test_claude_native_message_display_hook.
+# lifetime is user-visible latency. This journey times that lifetime for a
+# Python hook — isolated interpreter, module entrypoint, JSON payload on stdin —
+# which is the cost of ANY hook the bridge installs as a `python -m` command.
+#
+# It is NOT the per-chunk streaming path. The hooks that fire per chunk
+# (MessageDisplay) and per tool call (evaluate-policy) were deliberately moved
+# off the interpreter — a /bin/sh appender and a curl to the runner's relay
+# respectively — and tests pin that (`test_message_display_shell_command_round_trips`
+# asserts "python" is absent from the installed command). What still pays this
+# is the per-turn set (SessionStart / Stop / UserPromptSubmit / PreCompact /
+# Task*), the PostToolUse TodoWrite+TaskUpdate matchers, and the policy hook's
+# Python fallback when the relay is not yet up. So read this number as
+# "what a hook costs if it is Python", and as the standing argument for keeping
+# the hot paths off it — not as a per-chunk cost. The import-graph side is
+# pinned by tests/test_claude_native_message_display_hook.py.
 _HOOK_SPAWN_PAYLOAD = json.dumps(
     {
         "hook_event_name": "MessageDisplay",
@@ -905,7 +914,7 @@ async def _setup_hook_spawn(env: BenchEnvironment) -> JourneyContext:
 
 
 async def _measure_hook_spawn(env: BenchEnvironment, ctx: JourneyContext) -> None:
-    """Spawn the MessageDisplay hook once, as Claude Code does, and wait."""
+    """Spawn one Python hook subprocess and wait, as Claude Code would."""
     del env
     proc = await asyncio.create_subprocess_exec(
         sys.executable,
@@ -1082,7 +1091,12 @@ ALL_JOURNEYS: dict[str, Journey] = {
             measure=_measure_hook_spawn,
             setup=_setup_hook_spawn,
             teardown=_teardown_hook_spawn,
-            description="Spawn the per-chunk MessageDisplay hook exactly as Claude Code does.",
+            description=(
+                "Spawn one Python command hook (isolated interpreter, module "
+                "entrypoint) and time its whole lifetime — what any `python -m` "
+                "hook costs Claude's blocked TUI. Not the per-chunk path: that "
+                "one is a /bin/sh appender."
+            ),
         ),
         Journey(
             name="cli_startup",