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/403.json

@@ -2,20 +2,46 @@
   "$schema": "https://json-schema.org/draft/2020-12/schema",
   "$id": "https://git.eeqj.de/sneak/chat/schema/numerics/403.json",
   "title": "403 ERR_NOSUCHCHANNEL",
-  "description": "No such channel. RFC 1459 §6.1.",
+  "description": "No such channel. RFC 1459 \u00a76.1.",
   "$ref": "../message.json",
   "properties": {
-    "command": { "const": "403" },
-    "to": { "type": "string" },
+    "command": {
+      "const": "403"
+    },
+    "to": {
+      "type": "string"
+    },
     "params": {
       "type": "array",
-      "items": { "type": "string" },
+      "items": {
+        "type": "string"
+      },
       "description": "[channel_name]."
     },
-    "body": { "const": "No such channel" }
+    "body": {
+      "type": "array",
+      "items": {
+        "type": "string"
+      },
+      "description": "No such channel",
+      "maxItems": 1
+    }
   },
-  "required": ["command", "to", "params"],
+  "required": [
+    "command",
+    "to",
+    "params"
+  ],
   "examples": [
-    { "command": "403", "to": "alice", "params": ["#nonexistent"], "body": "No such channel" }
+    {
+      "command": "403",
+      "to": "alice",
+      "params": [
+        "#nonexistent"
+      ],
+      "body": [
+        "No such channel"
+      ]
+    }
   ]
 }