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.json

@@ -0,0 +1,46 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://git.eeqj.de/sneak/chat/schema/message.json",
+  "title": "IRC Message Envelope",
+  "description": "Base envelope for all messages. Mirrors IRC wire format (RFC 1459/2812) encoded as JSON over HTTP. The 'command' field carries either an IRC command name (PRIVMSG, JOIN, etc.) or a three-digit numeric reply code (001, 353, 433, etc.).",
+  "type": "object",
+  "properties": {
+    "id": {
+      "type": "integer",
+      "description": "Server-assigned message ID, monotonically increasing. Present on all server-originated messages."
+    },
+    "command": {
+      "type": "string",
+      "description": "IRC command name (PRIVMSG, JOIN, NICK, etc.) or three-digit numeric reply code (001, 353, 433, etc.).",
+      "examples": ["PRIVMSG", "JOIN", "001", "353", "433"]
+    },
+    "from": {
+      "type": "string",
+      "description": "Source — nick for user messages, server name for server messages. Equivalent to IRC prefix."
+    },
+    "to": {
+      "type": "string",
+      "description": "Target — channel (#name) or nick. Equivalent to first IRC parameter for most commands."
+    },
+    "params": {
+      "type": "array",
+      "items": { "type": "string" },
+      "description": "Additional parameters (used primarily by numeric replies). Equivalent to IRC middle parameters."
+    },
+    "body": {
+      "type": "string",
+      "description": "Message body / trailing parameter. Equivalent to IRC trailing parameter (after the colon)."
+    },
+    "ts": {
+      "type": "string",
+      "format": "date-time",
+      "description": "Server-assigned timestamp (ISO 8601). Not present in original IRC; added for HTTP transport."
+    },
+    "meta": {
+      "type": "object",
+      "description": "Extensible metadata (signatures, rich content hints, etc.). Not present in original IRC.",
+      "additionalProperties": true
+    }
+  },
+  "required": ["command"]
+}