docs(agents): bound code-comment length, not just its topic

The Conciseness rule constrained what a comment may say ("non-obvious intent") and never how
long — the only length bound sat under the docs clause, so a 20-line header on a new file
passed it. That is how one got written in a sibling repo.

The clause now names length and audience: a block arguing for its own change is a commit
message, and long evidence belongs in the project's agent docs.

Author: NamesMT

AGENTS.md

@@ -66,7 +66,8 @@ pnpm run release:preview        # print the changelog the next release would get
 ## Conciseness
 
 **Prune verbose, keep correctness** — code, comments, docs alike. A comment only for non-obvious
-intent; docs one idea per sentence, cut what would not change what a reader does. Delete history
+intent, and short — a block arguing for its own change is a commit message; docs one idea per
+sentence, cut what would not change what a reader does. Delete history
 `git log` already holds — keep the rule, not the story. Never drop a caveat to save a line.
 
 ## User-facing docs