Keep agent guidance in one root AGENTS.md (closes #31)

Writes down sneak's 2026-08-22 ruling: the in-repo memory rule that older vendored copies of `REPO_POLICIES.md` still carry was retired, not lost.

`prompts/REPO_POLICIES.md` gains one bullet after the files a new repo must contain: guidance for coding agents lives in one `AGENTS.md` at the repository root, never under a file or directory named after one agent tool, and never split into separate memory files. `AGENTS.md` joins the files allowed in the repo root, which would otherwise forbid it. Both checklists get the matching item; the existing-repo one says to move such a file's content into `AGENTS.md` and delete it.

The Dockerfile finding at the end of the issue is tracked in #90.

Model: opus-5-5

prompts/EXISTING_REPO_CHECKLIST.md

@@ -24,6 +24,10 @@ with your task.
 - [ ] `LICENSE` file exists and matches the README
 - [ ] `REPO_POLICIES.md` exists and version date is current — fetch from
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/prompts/REPO_POLICIES.md`
+- [ ] Guidance for coding agents, if the repo has any, is one `AGENTS.md` at the
+      root — never a file or directory named after one agent tool, such as
+      `CLAUDE.md` or `.claude/`, and never separate memory files. Move what any
+      such committed file says into `AGENTS.md` and delete it.
 - [ ] `.gitignore` is comprehensive (OS, editor, agent scratch, language
       artifacts, secrets) — fetch from
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/.gitignore` if missing.