refactor: model message schemas after IRC RFC 1459/2812

Replace c2s/s2c/s2s taxonomy with IRC-native structure:
- schema/commands/ — IRC command schemas (PRIVMSG, NOTICE, JOIN, PART,
  QUIT, NICK, TOPIC, MODE, KICK, PING, PONG)
- schema/numerics/ — IRC numeric reply codes (001-004, 322-323, 332,
  353, 366, 372-376, 401, 403, 433, 442, 482)
- schema/message.json — base envelope mapping IRC wire format to JSON

Messages use 'command' field with IRC command names or 3-digit numeric
codes. 'body' is a string (IRC trailing parameter), not object/array.
'from'/'to' map to IRC prefix and first parameter.

Federation uses the same IRC commands (no custom RELAY/LINK/SYNC).

Update README message format, command tables, and examples to match.

schema/commands/TOPIC.json

@@ -0,0 +1,17 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://git.eeqj.de/sneak/chat/schema/commands/TOPIC.json",
+  "title": "TOPIC",
+  "description": "Get or set channel topic. C2S: set topic (body present) or query (body absent). S2C: topic change notification. RFC 1459 §4.2.4.",
+  "$ref": "../message.json",
+  "properties": {
+    "command": { "const": "TOPIC" },
+    "from": { "type": "string", "description": "Nick that changed the topic (S2C)." },
+    "to": { "type": "string", "description": "Channel name.", "pattern": "^#[a-zA-Z0-9_-]+$" },
+    "body": { "type": "string", "description": "New topic text. Empty string clears the topic.", "maxLength": 512 }
+  },
+  "required": ["command", "to"],
+  "examples": [
+    { "command": "TOPIC", "from": "alice", "to": "#general", "body": "Welcome to the chat" }
+  ]
+}