[Docs] Do not add top-level files or folders without approval #150

User instruction (2026-10-03), as in the other repositories: never add a new file or folder at the repository root on your own; work goes inside existing modules and directories, temporary files under the git-ignored .tmp/; a new top-level entry is proposed (what, why, why no existing directory works) and waits for approval. AGENTS.md lists the approved root entries, and a test fails when a tracked root entry is not on the list.

Done when: AGENTS §2 has the rule and the list; api/tests/test_repo_layout.py checks tracked root entries against it.