feat: add IRC-style message protocol JSON schemas (draft 2020-12)

Add JSON Schema definitions for all message types:
- Base message envelope (message.schema.json)
- C2S: PRIVMSG, NOTICE, JOIN, PART, QUIT, NICK, MODE, TOPIC, KICK, PING, PUBKEY
- S2C: named commands + numeric reply codes (001, 002, 322, 353, 366, 372, 375, 376, 401, 403, 433)
- S2S: RELAY, LINK, UNLINK, SYNC, PING, PONG
- Schema index (schema/README.md)

All messages use IRC command names and numeric codes from RFC 1459/2812.
Bodies are always objects or arrays (never raw strings) to support
deterministic canonicalization (RFC 8785 JCS) and message signing.

schema/commands/PRIVMSG.json

@@ -0,0 +1,29 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://git.eeqj.de/sneak/chat/schema/commands/PRIVMSG.json",
+  "title": "PRIVMSG",
+  "description": "Send a message to a channel or user. C2S: client sends to server. S2C: server relays to recipients. RFC 1459 §4.4.1.",
+  "$ref": "../message.json",
+  "properties": {
+    "command": { "const": "PRIVMSG" },
+    "from": {
+      "type": "string",
+      "description": "Sender nick (set by server on S2C relay)."
+    },
+    "to": {
+      "type": "string",
+      "description": "Target: #channel or nick.",
+      "examples": ["#general", "alice"]
+    },
+    "body": {
+      "type": "string",
+      "description": "Message text.",
+      "minLength": 1
+    }
+  },
+  "required": ["command", "to", "body"],
+  "examples": [
+    { "command": "PRIVMSG", "from": "bob", "to": "#general", "body": "hello world" },
+    { "command": "PRIVMSG", "from": "bob", "to": "alice", "body": "hey" }
+  ]
+}