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/commands/KICK.json

@@ -6,15 +6,8 @@
   "$ref": "../message.json",
   "properties": {
     "command": { "const": "KICK" },
-    "from": {
-      "type": "string",
-      "description": "Nick that performed the kick."
-    },
-    "to": {
-      "type": "string",
-      "description": "Channel name.",
-      "pattern": "^#[a-zA-Z0-9_-]+$"
-    },
+    "from": { "type": "string", "description": "Nick that performed the kick." },
+    "to": { "type": "string", "description": "Channel name.", "pattern": "^#[a-zA-Z0-9_-]+$" },
     "params": {
       "type": "array",
       "items": { "type": "string" },
@@ -23,12 +16,14 @@
       "maxItems": 1
     },
     "body": {
-      "type": "string",
-      "description": "Optional kick reason."
+      "type": "array",
+      "items": { "type": "string" },
+      "description": "Optional kick reason.",
+      "maxItems": 1
     }
   },
   "required": ["command", "to", "params"],
   "examples": [
-    { "command": "KICK", "from": "op1", "to": "#general", "params": ["troll"], "body": "Behave" }
+    { "command": "KICK", "from": "op1", "to": "#general", "params": ["troll"], "body": ["Behave"] }
   ]
 }