refactor: model message schemas after IRC RFC 1459/2812

Replace c2s/s2c/s2s taxonomy with IRC-native structure:
- schema/commands/ — IRC command schemas (PRIVMSG, NOTICE, JOIN, PART,
  QUIT, NICK, TOPIC, MODE, KICK, PING, PONG)
- schema/numerics/ — IRC numeric reply codes (001-004, 322-323, 332,
  353, 366, 372-376, 401, 403, 433, 442, 482)
- schema/message.json — base envelope mapping IRC wire format to JSON

Messages use 'command' field with IRC command names or 3-digit numeric
codes. 'body' is a string (IRC trailing parameter), not object/array.
'from'/'to' map to IRC prefix and first parameter.

Federation uses the same IRC commands (no custom RELAY/LINK/SYNC).

Update README message format, command tables, and examples to match.

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"]
+}