Make the schema files match the server (closes #218)

Each file in schema/ now describes what the HTTP API on next sends and
accepts. A numeric reply has its name in command and its number in code.
body is always an array of strings. MODE, KICK and JOIN carry their
values in body, and TOPIC needs a non-empty body. PING takes no token,
and its PONG is the HTTP response. A channel name is limited to 63
characters, and the leading # is optional from a client.

PUBKEY.json is deleted, since the server has no such command. The
section of schema/README.md that listed the differences is removed,
and the sentences it made false are corrected.

Model: opus-5-5

schema/commands/PING.json

@@ -2,19 +2,13 @@
   "$schema": "https://json-schema.org/draft/2020-12/schema",
   "$id": "https://sneak.berlin/go/neoirc/schema/commands/PING.json",
   "title": "PING",
-  "description": "Keepalive. C2S or S2S. Server responds with PONG. RFC 1459 §4.6.2.",
+  "description": "A check that the server still answers. A client sends it. The server ignores every field but 'command'. Its answer is a PONG in the HTTP response. RFC 1459 §4.6.2.",
   "$ref": "../message.json",
   "properties": {
-    "command": { "const": "PING" },
-    "body": {
-      "type": "array",
-      "items": { "type": "string" },
-      "description": "Opaque token to be echoed in PONG (single-element array).",
-      "maxItems": 1
-    }
+    "command": { "const": "PING" }
   },
   "required": ["command"],
   "examples": [
-    { "command": "PING", "body": ["1707580000"] }
+    { "command": "PING" }
   ]
 }