docs(18-01): complete auto-take-ownership settings foundation plan

- 18-01-SUMMARY.md: plan execution summary
- STATE.md: progress updated to 98%, decisions recorded, stopped-at updated
- ROADMAP.md: phase 18 marked in-progress (1/2 summaries)
- REQUIREMENTS.md: OWN-01 marked complete

.planning/STATE.md

@@ -3,14 +3,14 @@ gsd_state_version: 1.0
 milestone: v2.3
 milestone_name: Tenant Management & Report Enhancements
 status: planning
-stopped_at: Completed 17-02-PLAN.md
-last_updated: "2026-04-09T11:13:35.417Z"
+stopped_at: Completed 18-01-PLAN.md
+last_updated: "2026-04-09T12:25:36.302Z"
 last_activity: 2026-04-09 — Roadmap created for v2.3 (phases 15-19)
 progress:
   total_phases: 5
   completed_phases: 3
-  total_plans: 6
-  completed_plans: 6
+  total_plans: 8
+  completed_plans: 7
 ---
 
 # Project State
@@ -71,6 +71,9 @@ Decisions are logged in PROJECT.md Key Decisions table.
 - [Phase 17]: Static helpers IsAadGroup/ExtractAadGroupId/StripClaims declared internal to enable unit testing via InternalsVisibleTo without polluting public API
 - [Phase 17]: Graph client created lazily on first AAD group encountered to avoid unnecessary auth overhead for groups with no nested AAD members
 - [Phase 17]: groupMembers optional param in HtmlExportService — null produces identical pre-Phase-17 output; ISharePointGroupResolver injected as optional last param in PermissionsViewModel; resolution failure degrades gracefully with LogWarning
+- [Phase 18-auto-take-ownership]: OwnershipElevationService uses Tenant.SetSiteAdmin from PnP.Framework
+- [Phase 18-auto-take-ownership]: WasAutoElevated last positional param with default=false preserves all existing PermissionEntry callsites
+- [Phase 18-auto-take-ownership]: AutoTakeOwnership ViewModel setter uses fire-and-forget pattern matching DataFolder
 
 ### Pending Todos
 
@@ -82,7 +85,7 @@ None.
 
 ## Session Continuity
 
-Last session: 2026-04-09T11:11:03.823Z
-Stopped at: Completed 17-02-PLAN.md
+Last session: 2026-04-09T12:25:29.455Z
+Stopped at: Completed 18-01-PLAN.md
 Resume file: None
 Next step: `/gsd:plan-phase 15`