docs: fence residual TS backend as non-authoritative (Work: wi_01KYFN6993PMG8WD00Q51AE231)

AGENTS.md

@@ -25,3 +25,19 @@ Local truth: `PROJECT.md`, `.doctrine/project.json` when present.
 
 - Prefer the **narrowest** affected check before full workspace runs.
 - Report layers honestly: local diff · trunk FF · deploy · prod proof (do not collapse).
+
+## Backend false-authority fence
+
+Work: wi_01KYFN6993PMG8WD00Q51AE231
+
+If this repository has completed a **Rust backend** cutover:
+
+1. Production backend behavior authority is the Rust crate/binary/service path declared in `sylphx.toml` / deploy manifests / package `bin` native path / Docker ENTRYPOINT.
+2. Residual TypeScript service trees or alternate TS engines are **not** product authority unless explicitly proven still on the live path.
+3. Do not "fix production" by editing residual TypeScript and assuming deploy/runtime will pick it up.
+4. Prefer deleting residual TS backend trees after Rust sole proof; keep history in Git.
+5. Intentional TypeScript frontends, npm packaging wrappers, and native-binding surfaces may remain.
+
+### Repo-specific note
+
+Rust binary is runtime authority; TS/npm packaging is not an alternate backend implementation.