docs(benchmark): R6 holds the push path to the LLM tier; measure hook as shipped and hook + hunter

The pre-push hook has no LLM detection tier today: push/assistance.py only
selects a witness for already-admitted defects and cannot add a finding. The
hunter lives in scan --mode deep. The maintainer's call is that the LLM tier is
required on pre-push, so R6 now measures two arms over the same changes:
(a) the hook as shipped (quick rules + engine), (b) the hook plus the hunter
arm run off-cluster. (b) is the number the tool is held to; the gap is product
work in pre-push-safety-net, not a benchmark artefact.

Adds group 6 (hunter arm runner + combine + offline test) to tasks, aligns the
brief and design, and labels the overnight replay as arm (a).

Co-Authored-By: Claude Fable 5.1 <[email protected]>

.kiro/specs/valid-detection-benchmark/brief.md

@@ -13,8 +13,10 @@ A pre-push security check must answer two questions:
 1. **If I'm about to push a real vulnerability, does it stop me?**
 2. **If my change is fine, does it leave me alone?**
 
-The benchmark runs the **whole tool** as a developer uses it at push time. Quick rules and the engine are on,
-as is the LLM-guided arm. The repositories are ones the tool has **never seen**.
+The benchmark measures the **push path** as a developer uses it. The LLM detection tier (the hunter) is
+required on pre-push. Today the shipped hook runs quick rules and the engine only, so the benchmark
+reports the hook as shipped and the hook plus the hunter arm separately until the hunter is wired in
+(see `requirements.md` R6). The repositories are ones the tool has **never seen**.
 
 ## What makes the data valid