Add guideline for brevity in code and comments

Added a guideline emphasizing brevity in code, comments, and commit messages.

Author: bep

AGENTS.md

@@ -8,6 +8,7 @@
 * Avoid global state at (almost) all cost.
 * This is a project with a long history; assume that a similiar problem has been solved before, look hard for helper functions before creating new ones.
 * In tests, use `qt` matchers (e.g. `b.Assert(err, qt.ErrorMatches, ...)`) instead of raw `if`/`t.Fatal` checks.
+* Brevity is good. This applies to code, comments and commit messages. Don't write a novel.
 * Use `./check.sh ./somepackage/...` when iterating.
 * Use `./check.sh` when you're done.