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/s2c/376.schema.json

@@ -0,0 +1,24 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://git.eeqj.de/sneak/chat/schema/s2c/376.schema.json",
+  "title": "376 RPL_ENDOFMOTD (S2C)",
+  "description": "End of MOTD",
+  "$ref": "../message.schema.json",
+  "properties": {
+    "command": {
+      "const": "376"
+    },
+    "body": {
+      "type": "array",
+      "items": {
+        "type": "string"
+      },
+      "description": "Response lines"
+    }
+  },
+  "required": [
+    "command",
+    "to",
+    "body"
+  ]
+}