fix(comm): use JSON (stdlib) for shared_context; update docs

Switch from PyYAML dependency to json module for maximum portability.
File is now wizards/shared_context.json — same schema, JSON encoding.
wizard-summon.py rewritten to use json.loads/dumps (no external dep).
Docs updated accordingly.

wizards/shared_context.json

@@ -0,0 +1,46 @@
+{
+  "version": "1.0",
+  "updated_at": "2026-04-26T00:00:00Z",
+  "active_summon": {
+    "summon_id": null,
+    "priority": null,
+    "topic": null,
+    "summoner": null,
+    "summoned_at": null,
+    "deadline": null,
+    "status": null,
+    "acknowledgements": {
+      "timmy": null,
+      "allegro": null,
+      "bezalel": null,
+      "ezra": null
+    }
+  },
+  "wizard_status": {
+    "timmy": {
+      "status": "idle",
+      "last_seen": null,
+      "current_task": null,
+      "notes": null
+    },
+    "allegro": {
+      "status": "idle",
+      "last_seen": null,
+      "current_task": null,
+      "notes": null
+    },
+    "bezalel": {
+      "status": "idle",
+      "last_seen": null,
+      "current_task": null,
+      "notes": null
+    },
+    "ezra": {
+      "status": "idle",
+      "last_seen": null,
+      "current_task": null,
+      "notes": null
+    }
+  },
+  "message_log": []
+}