docs(17): create phase plan

.planning/STATE.md

@@ -5,10 +5,10 @@ milestone_name: Operability & Polish
 current_phase: 999.1
 current_phase_name: BACKLOG
 status: executing
-stopped_at: Phase 17 context gathered
-last_updated: "2026-06-18T00:37:02.287Z"
+stopped_at: Phase 17 UI-SPEC approved
+last_updated: "2026-06-18T15:36:11.741Z"
 last_activity: 2026-06-18
-last_activity_desc: Phase 19 complete, transitioned to Phase 999.1
+last_activity_desc: Phase 17 planning complete
 progress:
   total_phases: 26
   completed_phases: 11
@@ -31,7 +31,7 @@ See: .planning/PROJECT.md (updated 2026-06-16)
 Phase: 999.1 — Treat Fastmail as one calendar provider; framework supports adding more providers (BACKLOG)
 Plan: Not started
 Status: Phase 19 shipped — PR #23
-Last activity: 2026-06-17
+Last activity: 2026-06-18 — Phase 17 planning complete
 
 ### ✅ Resolved Checkpoint — Phase 15 Plan 15-03 Task 2 (human-action)
 
@@ -272,9 +272,9 @@ Recent decisions affecting current work:
 
 ## Session Continuity
 
-Last session: 2026-06-18T00:37:02.272Z
-Stopped at: Phase 17 context gathered
-Resume file: .planning/phases/17-ui-optimization-polish/17-CONTEXT.md
+Last session: 2026-06-18T15:00:30.986Z
+Stopped at: Phase 17 UI-SPEC approved
+Resume file: .planning/phases/17-ui-optimization-polish/17-UI-SPEC.md
 
 ## Operator Next Steps