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/numerics/002.json

@@ -0,0 +1,20 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$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.",
+  "$ref": "../message.json",
+  "properties": {
+    "command": { "const": "002" },
+    "to": { "type": "string" },
+    "body": {
+      "type": "array",
+      "items": { "type": "string" },
+      "description": "Host info lines."
+    }
+  },
+  "required": ["command", "to", "body"],
+  "examples": [
+    { "command": "002", "to": "alice", "body": ["Your host is chat.example.com, running version 0.1.0"] }
+  ]
+}