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/NOTICE.json

@@ -0,0 +1,23 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://git.eeqj.de/sneak/chat/schema/commands/NOTICE.json",
+  "title": "NOTICE",
+  "description": "Send a notice. Like PRIVMSG but must not trigger automatic replies. RFC 1459 §4.4.2.",
+  "$ref": "../message.json",
+  "properties": {
+    "command": { "const": "NOTICE" },
+    "from": { "type": "string" },
+    "to": {
+      "type": "string",
+      "description": "Target: #channel, nick, or * (global)."
+    },
+    "body": {
+      "type": "string",
+      "description": "Notice text."
+    }
+  },
+  "required": ["command", "to", "body"],
+  "examples": [
+    { "command": "NOTICE", "from": "server.example.com", "to": "*", "body": "Server restarting in 5 minutes" }
+  ]
+}