Replace deprecated AGENTS.md with agent contract

Replace the legacy deprecation notice with a concrete generic agent contract. The file now documents the canonical agent prompt location (/usr/share/mios/ai/system.md), a host-local override (/etc/mios/ai/system-prompt.md), a repo-root fallback (/system-prompt.md), and a short behavior checklist for generic agents. It also includes a sanitization reminder about removing corporate references and chat metadata from persisted artifacts. This provides a machine-readable redirector and guidance for agents that lack dedicated rules files.

Author: Kabuki94

AGENTS.md

@@ -1,2 +1,23 @@
-# DEPRECATED: AGENTS.md
-This file is legacy. All MiOS Agent Surface contracts have been merged into the Single Source of Truth: [INDEX.md](./INDEX.md).
+# AGENTS.md
+
+> Generic agent contract following the AGENTS.md standard.
+> Read by Codex, Cursor, Aider, Continue.dev, and other agents that don't
+> have a dedicated rules file. Redirector stub.
+>
+> Canonical agent prompt: `/usr/share/mios/ai/system.md`.
+
+## Behavior contract
+
+1. Load `/usr/share/mios/ai/system.md` first.
+2. Apply `/etc/mios/ai/system-prompt.md` as host-local override if present.
+3. This file carries no tool-specific deltas; generic agents follow the
+   canonical prompt verbatim.
+
+If `/usr/share/mios/ai/system.md` is unreachable, fall back to
+`/system-prompt.md` in the repo root.
+
+## Sanitization reminder
+
+Per `/usr/share/mios/ai/system.md` §6: no corporate entity references in
+persisted artifacts, no chat metadata, OpenAI API-compliant minimal form.
+This applies to all output you persist to this repository.