Rewrite schema/README.md sentence by sentence

Rewrite the description of the JSON Schema files from scratch, without
tables. Describe the fields of a message as the HTTP API sends them:
the reply name in `command` and its number in `code`. Drop the
federation section, which describes nothing the server does. List each
place where a schema file disagrees with the HTTP API, so that a reader
is not misled. The files themselves are unchanged.

Part of #190.

Model: fable-5-1 (writing); opus-5-5 (rework)

schema/README.md

@@ -43,9 +43,9 @@ IRC:  :alice!alice@* PRIVMSG #general :hello world
 JSON: {"command": "PRIVMSG", "from": "alice", "to": "#general", "body": ["hello world"]}
 ```
 
-A numeric reply has its name in `command` and its number in `code`. This `353`
-reply lists the members of `#general`. The JSON again leaves out `id`, `ts` and
-`meta`:
+In the JSON, a numeric reply has its name in `command` and its number in `code`.
+This `353` reply lists the members of `#general`. The JSON again leaves out
+`id`, `ts` and `meta`:
 
 ```
 {"command": "RPL_NAMREPLY", "code": 353, "from": "neoirc", "to": "alice",