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/message.schema.json

@@ -0,0 +1,67 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://git.eeqj.de/sneak/chat/schema/message.schema.json",
+  "title": "Chat Message Envelope",
+  "description": "Base message envelope. Bodies MUST be objects or arrays (never raw strings) for deterministic canonicalization (RFC 8785 JCS) and signing.",
+  "type": "object",
+  "required": [
+    "command"
+  ],
+  "properties": {
+    "id": {
+      "type": "string",
+      "format": "uuid",
+      "description": "Server-assigned UUID"
+    },
+    "ts": {
+      "type": "string",
+      "format": "date-time",
+      "description": "Server-assigned timestamp (ISO 8601)"
+    },
+    "command": {
+      "type": "string",
+      "description": "IRC command name or numeric reply code"
+    },
+    "from": {
+      "type": "string",
+      "description": "Sender nick or server name"
+    },
+    "to": {
+      "type": "string",
+      "description": "Destination: channel (#foo) or nick"
+    },
+    "params": {
+      "type": "array",
+      "items": {
+        "type": "string"
+      },
+      "description": "Additional IRC-style parameters"
+    },
+    "body": {
+      "oneOf": [
+        {
+          "type": "array"
+        },
+        {
+          "type": "object"
+        }
+      ],
+      "description": "Message body (array or object, never raw string)"
+    },
+    "meta": {
+      "type": "object",
+      "description": "Extensible metadata",
+      "properties": {
+        "sig": {
+          "type": "string",
+          "description": "Cryptographic signature (base64)"
+        },
+        "alg": {
+          "type": "string",
+          "description": "Signature algorithm (e.g. ed25519)"
+        }
+      }
+    }
+  },
+  "additionalProperties": false
+}