docs: add plugin-v2 scope isolation test plan

Author: Thomas Tupper

SCOPE_ISOLATION_TEST_PLAN.md

@@ -0,0 +1,22 @@
+# Scope Isolation Test Plan (plugin-v2)
+
+## Goal
+Prove strict separation for:
+- session memory (`run_id`)
+- long-term memory (`user_id`)
+- merged `all`
+
+## Cases
+1. Write session marker only, verify:
+   - found in `scope=session`
+   - not found in `scope=long-term`
+2. Write long-term marker only, verify:
+   - found in `scope=long-term`
+   - not found in `scope=session`
+3. Write one marker in each lane, verify:
+   - both appear in `scope=all`
+   - dedupe stable when identical text appears in both lanes
+
+## Pass Criteria
+- zero cross-lane leakage for cases 1 and 2
+- deterministic merged output in case 3