docs(12.2): create phase plan

Five sequential plans: foundations (deferred_bindings, attach.Store,
lagoon.Date/TimeOfDay, purge), cabana datepicker and fileupload, relation
child CRUD with deferral, admin SPA, and unit and security tests.
Adds D-22..D-24 from the plan-count checkpoint and the pattern map.

.planning/state.json

@@ -68,15 +68,10 @@
       "name": "summercms.io Alpha 0.1 landing page on SummerCMS",
       "status": "in_progress"
     },
-    {
-      "number": "11.3",
-      "name": "Newsletter plugin and signup on summercms.io",
-      "status": "pending"
-    },
     {
       "number": "12",
       "name": "Płytarium API — Collections and Albums",
-      "status": "pending"
+      "status": "in_progress"
     },
     {
       "number": "13",
@@ -96,8 +91,8 @@
   ],
   "next": {
     "command": "/gsd:progress --next",
-    "label": "Advance to the next step (verify)",
-    "reason": "Phase 11.2 of 20 · ready to verify"
+    "label": "Advance to the next step",
+    "reason": "Phase 12.2 of 21 · executing"
   },
-  "updated_at": "2026-10-01T21:19:02.996Z"
+  "updated_at": "2026-10-02T14:53:28.681Z"
 }