docs(19-01): complete reference item model and tags schema plan

- Add 19-01-SUMMARY.md with execution results
- Update STATE.md with phase 19 position and decisions
- Update ROADMAP.md with plan progress

.planning/STATE.md

@@ -3,7 +3,7 @@ gsd_state_version: 1.0
 milestone: v1.3
 milestone_name: Research & Decision Tools
 status: planning
-stopped_at: Completed 18-05-PLAN.md
+stopped_at: Completed 19-01-PLAN.md
 last_updated: "2026-04-05T11:22:25.312Z"
 last_activity: 2026-04-05
 progress:
@@ -25,12 +25,12 @@ See: .planning/PROJECT.md (updated 2026-04-03)
 
 ## Current Position
 
-Phase: 18 of 18 (PostgreSQL Migration)
-Plan: Not started
-Status: Ready to plan
+Phase: 19 of 19 (Reference Item Model & Tags Schema)
+Plan: 1 of 3
+Status: Executing
 Last activity: 2026-04-05
 
-Progress: [----------] 0% (v2.0 milestone)
+Progress: [#---------] 3% (v2.0 milestone)
 
 ## Performance Metrics
 
@@ -55,6 +55,9 @@ Key decisions made during v2.0 planning:
 - Separate globalItems table — not a flag on user items table
 - Single-user SQLite mode diverges at v2.0 boundary
 - [Phase 18]: Profile data loaded via usePublicProfile(userId) not /auth/me extension
+- [Phase 19]: Direct globalItemId FK on items replaces itemGlobalLinks junction table
+- [Phase 19]: Data migration SQL: UPDATE items before DROP TABLE item_global_links
+- [Phase 19]: Flat tags system without type categorization per D-16
 
 ### Pending Todos
 
@@ -67,6 +70,6 @@ None active.
 
 ## Session Continuity
 
-Last session: 2026-04-05T11:20:56.920Z
-Stopped at: Completed 18-05-PLAN.md
+Last session: 2026-04-05T18:28:00Z
+Stopped at: Completed 19-01-PLAN.md
 Resume file: None