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/002.json

@@ -2,19 +2,35 @@
   "$schema": "https://json-schema.org/draft/2020-12/schema",
   "$id": "https://git.eeqj.de/sneak/chat/schema/numerics/002.json",
   "title": "002 RPL_YOURHOST",
-  "description": "Server host info sent after session creation. RFC 2812 §5.1.",
+  "description": "Server host info sent after session creation. RFC 2812 \u00a75.1.",
   "$ref": "../message.json",
   "properties": {
-    "command": { "const": "002" },
-    "to": { "type": "string" },
+    "command": {
+      "const": "002"
+    },
+    "to": {
+      "type": "string"
+    },
     "body": {
       "type": "array",
-      "items": { "type": "string" },
+      "items": {
+        "type": "string"
+      },
       "description": "Host info lines."
     }
   },
-  "required": ["command", "to", "body"],
+  "required": [
+    "command",
+    "to",
+    "body"
+  ],
   "examples": [
-    { "command": "002", "to": "alice", "body": ["Your host is chat.example.com, running version 0.1.0"] }
+    {
+      "command": "002",
+      "to": "alice",
+      "body": [
+        "Your host is chat.example.com, running version 0.1.0"
+      ]
+    }
   ]
 }