docs: add memory activation gate checklist

docs/security/user-data-isolation-activation-gate.md

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+# User Data Isolation Activation Gate
+
+## Purpose
+
+Define the final conditions required before enabling memory search or cross-agent memory recall.
+
+## Current State
+
+```text
+memory search: disabled
+isolation gate: enforced
+```
+
+## Activation Checklist
+
+Memory search may be enabled only when all checks pass.
+
+### Storage isolation
+
+- [ ] Records are stored under scoped namespaces.
+- [ ] No global shared memory path is used for production search.
+- [ ] Every record has required scope metadata.
+
+### Read isolation
+
+- [ ] Scope envelope is attached to every search request.
+- [ ] Provider filters before ranking.
+- [ ] Out-of-scope records never reach summarization.
+- [ ] Provenance is returned for every accepted record.
+
+### Audit
+
+- [ ] Isolation audit reports `healthy`.
+- [ ] Denied records count is zero.
+- [ ] Invalid records count is zero.
+- [ ] Safe-mode path is tested.
+
+### Provider
+
+Required contract:
+
+```json
+{
+  "scope_filter_before_ranking": true,
+  "provenance_required": true,
+  "allow_global_index": false
+}
+```
+
+### Owner Gate
+
+Activation requires explicit approval because memory/search changes the data exposure boundary.
+
+## Failure Behavior
+
+If any check fails:
+
+```text
+memory remains disabled
+        +
+audit event generated
+        +
+safe-mode available for autonomous operations
+```
+
+## Non-Goals
+
+This gate does not:
+
+- grant RBAC permissions;
+- replace application authorization;
+- allow global recall for convenience;
+- bypass tenant/user/project boundaries.
+
+## Release Rule
+
+User-data isolation is a release blocker, not an optimization item.