docs: add PUBKEY schema for signing key distribution

Dedicated JSON Schema for the PUBKEY command — announces/relays
user signing public keys. Body is a structured object with alg
and key fields. Already documented in README; this adds the
schema file and index entry.

schema/commands/PUBKEY.json

@@ -0,0 +1,37 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://git.eeqj.de/sneak/chat/schema/commands/PUBKEY.json",
+  "title": "PUBKEY",
+  "description": "Announce or relay a user's public signing key. C2S: client announces key to channel or server. S2C: server relays to channel members. Protocol extension (not in RFC 1459). Body is a structured object (not an array) containing the key material.",
+  "$ref": "../message.json",
+  "properties": {
+    "command": { "const": "PUBKEY" },
+    "from": { "type": "string", "description": "Nick announcing the key (set by server on relay)." },
+    "to": {
+      "type": "string",
+      "description": "Target: #channel to announce to channel members, or omit for server-wide announcement."
+    },
+    "body": {
+      "type": "object",
+      "description": "Key material.",
+      "properties": {
+        "alg": {
+          "type": "string",
+          "description": "Key algorithm.",
+          "enum": ["ed25519"]
+        },
+        "key": {
+          "type": "string",
+          "description": "Base64-encoded public key."
+        }
+      },
+      "required": ["alg", "key"],
+      "additionalProperties": false
+    }
+  },
+  "required": ["command", "body"],
+  "examples": [
+    { "command": "PUBKEY", "from": "alice", "body": { "alg": "ed25519", "key": "base64-encoded-pubkey" } },
+    { "command": "PUBKEY", "from": "alice", "to": "#general", "body": { "alg": "ed25519", "key": "base64-encoded-pubkey" } }
+  ]
+}