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/PART.json

@@ -0,0 +1,27 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://git.eeqj.de/sneak/chat/schema/commands/PART.json",
+  "title": "PART",
+  "description": "Leave a channel. C2S: request to leave. S2C: notification that a user left. RFC 1459 §4.2.2.",
+  "$ref": "../message.json",
+  "properties": {
+    "command": { "const": "PART" },
+    "from": {
+      "type": "string",
+      "description": "Nick that left (S2C only)."
+    },
+    "to": {
+      "type": "string",
+      "description": "Channel name.",
+      "pattern": "^#[a-zA-Z0-9_-]+$"
+    },
+    "body": {
+      "type": "string",
+      "description": "Optional part reason."
+    }
+  },
+  "required": ["command", "to"],
+  "examples": [
+    { "command": "PART", "from": "alice", "to": "#general", "body": "later" }
+  ]
+}