refactor: structured body (array|object, never string) for canonicalization

Message bodies are always arrays of strings (text lines) or objects
(structured data like PUBKEY). Never raw strings. This enables:
- Multiline messages without escape sequences
- Deterministic JSON canonicalization (RFC 8785 JCS) for signing
- Structured data where needed

Update all schemas: body fields use array type with string items.
Update message.json envelope: body is oneOf[array, object], id is UUID.
Update README: message envelope table, examples, and canonicalization docs.
Update schema/README.md: field types, examples with array bodies.

schema/numerics/376.json

@@ -2,12 +2,26 @@
   "$schema": "https://json-schema.org/draft/2020-12/schema",
   "$id": "https://git.eeqj.de/sneak/chat/schema/numerics/376.json",
   "title": "376 RPL_ENDOFMOTD",
-  "description": "End of MOTD. RFC 2812 §5.1.",
+  "description": "End of MOTD. RFC 2812 \u00a75.1.",
   "$ref": "../message.json",
   "properties": {
-    "command": { "const": "376" },
-    "to": { "type": "string" },
-    "body": { "const": "End of /MOTD command" }
+    "command": {
+      "const": "376"
+    },
+    "to": {
+      "type": "string"
+    },
+    "body": {
+      "type": "array",
+      "items": {
+        "type": "string"
+      },
+      "description": "End of /MOTD command",
+      "maxItems": 1
+    }
   },
-  "required": ["command", "to"]
+  "required": [
+    "command",
+    "to"
+  ]
 }