#589 extension push error for disallowed file types doesn't mention binaries field

When pushing an extension with a binary file in additionalFiles, the error message is:

File extension "" is not allowed. Allowed: .ts, .json, .md, .yaml, .yml, .txt

Two problems:

1. CLI error message doesn't mention that the binaries manifest field exists and is exempt from the allowlist. A user hitting this error has no way to discover the correct solution without searching issues.

2. Skill/agent docs (extension guide, references/extension/guide.md) have zero mention of binaries. The only reference is a parenthetical in a table row in publishing.md: "binaries exempt". There's no section explaining the field, its schema, resolution behavior, or when to use it vs additionalFiles.

Suggested fix for the CLI error:

File extension "" is not allowed in additionalFiles. Allowed: .ts, .json, .md, .yaml, .yml, .txt.
Hint: use the `binaries` manifest field for executable files (exempt from extension allowlist).

Related: swamp-club issue #305 shipped the feature but the docs gap it describes may not have been fully addressed in the skill layer.