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/PART.json

@@ -8,10 +8,15 @@
     "command": { "const": "PART" },
     "from": { "type": "string", "description": "Nick that left (S2C)." },
     "to": { "type": "string", "description": "Channel name.", "pattern": "^#[a-zA-Z0-9_-]+$" },
-    "body": { "type": "string", "description": "Optional part reason." }
+    "body": {
+      "type": "array",
+      "items": { "type": "string" },
+      "description": "Optional part reason.",
+      "maxItems": 1
+    }
   },
   "required": ["command", "to"],
   "examples": [
-    { "command": "PART", "from": "alice", "to": "#general", "body": "later" }
+    { "command": "PART", "from": "alice", "to": "#general", "body": ["later"] }
   ]
 }