docs: simplify BGM format description

docs/bgm-format.md

@@ -1,16 +1,10 @@
 # Board Game Manifest — Technical Reference
 
-> The concrete behavior of the board game manifest (bgm) format. This is the
-> companion to the informal [`bgmanifest.md`](./bgmanifest.md) spec; where the
-> two disagree, this document is authoritative.
+> The concrete behavior of the board game manifest (bgm) format.
 >
 > Definitions can live in JSON/YAML/TOML files or in markdown code blocks. In
 > codeblock mode, each code block is a virtual definition file, named relative
 > to the current markdown file.
->
-> Assume every `.md` file in the package is preprocessed for virtual files:
-> its code blocks are extracted and treated as definitions before any other
-> resolution happens.
 
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