[Docs] (AGENTS.md, CLAUDE.md, CONTRIBUTING.md): add rules for commits, comments and files

AGENTS.md

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+# Notes for coding agents
+
+Read `CONTRIBUTING.md` before changing anything. Every rule in it applies to agents.
+
+@CONTRIBUTING.md
+
+- Before each commit:
+  1. Stage exactly what you mean to commit (no `git commit -a`).
+  2. Print the scope with `python3 scripts/check_text_hygiene.py scope`.
+  3. Write the message as UTF-8 without a byte-order mark, to a file outside the repository.
+  4. Check it with `python3 scripts/check_text_hygiene.py message <file>`.
+  5. Commit with `git commit -F <file>` once the check passes.
+- On Windows, run `python` instead of `python3`.
+- Do not copy the format of older commits. Much of the history before October 2026 breaks these rules.
+- When you brief a sub-agent, give it the exact commit format and these rules, not a paraphrase.
+- Keep working papers (briefs, reviews, scratch measurements, logs) outside the repository.
+- Add no `Co-Authored-By` or other trailers.