AGENTS.md: sync with instructions of other repositories (#282)

Bring in the lesson recorded in the other repositories' AGENTS.md after it was merged here: only add information that isn't obvious from the repository.

Based on https://github.com/fphammerle/free-disk/blob/356731a084bb99e163f400833c198041664a154c/AGENTS.md and https://github.com/fphammerle/systemctl-mqtt/blob/985c7a418c277d45a1e51b07e489de423e1bffcc/AGENTS.md

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

AGENTS.md

@@ -6,6 +6,8 @@
   record the lesson in `AGENTS.md` **automatically**, without being asked, so it isn't repeated.
 - If a mistake recurs although a rule for it already exists, make that rule stricter and more
   concrete (e.g. a mandatory check before acting) instead of just restating it.
+- When writing or updating `AGENTS.md`, only add information that isn't already obvious from the
+  repository itself (e.g. Git LFS tracking is visible in `.gitattributes` and marked on GitHub).
 
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