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/PRIVMSG.json

@@ -8,11 +8,17 @@
     "command": { "const": "PRIVMSG" },
     "from": { "type": "string", "description": "Sender nick (set by server on relay)." },
     "to": { "type": "string", "description": "Target: #channel or nick.", "examples": ["#general", "alice"] },
-    "body": { "type": "string", "description": "Message text.", "minLength": 1 }
+    "body": {
+      "type": "array",
+      "items": { "type": "string" },
+      "description": "Message lines. One string per line.",
+      "minItems": 1
+    }
   },
   "required": ["command", "to", "body"],
   "examples": [
-    { "command": "PRIVMSG", "from": "bob", "to": "#general", "body": "hello world" },
-    { "command": "PRIVMSG", "from": "bob", "to": "alice", "body": "hey" }
+    { "command": "PRIVMSG", "from": "bob", "to": "#general", "body": ["hello world"] },
+    { "command": "PRIVMSG", "from": "bob", "to": "#general", "body": ["line one", "line two"] },
+    { "command": "PRIVMSG", "from": "bob", "to": "alice", "body": ["hey"], "meta": { "sig": "base64...", "alg": "ed25519" } }
   ]
 }