Document AndChat fork policy

FORK.md

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+# AndChat OpenIM Fork Policy
+
+This fork backs the AndChat in-house OpenIM deployment.
+
+## Remotes
+
+```text
+upstream = https://github.com/openimsdk/open-im-server.git
+origin   = git@github.com:yuanlu-and/open-im-server.git
+```
+
+## Branches
+
+```text
+upstream/main
+- pristine OpenIM source
+
+origin/main
+- mirror of upstream/main
+
+origin/andchat-prod
+- production branch for the AndChat OpenIM deployment
+```
+
+Keep `main` close to upstream. Put AndChat deployment patches on `andchat-prod`.
+
+## What Belongs In This Fork
+
+Acceptable changes:
+
+- deployment and configuration defaults
+- Docker/build adjustments
+- observability and logging
+- small OpenIM bug fixes
+- chat-engine behavior that must live inside OpenIM
+- narrow extension points that can be proposed upstream
+
+Avoid adding:
+
+- phone OTP auth
+- phone-number discovery
+- app profiles
+- billing
+- AI memory or retrieval logic
+- product-specific permissions
+- reaction source-of-truth tables unless reactions become OpenIM-native
+
+Those features belong in `andchat-backend`.
+
+## Current Divergence
+
+No runtime behavior changes yet.
+
+This file is the initial fork policy document for the `andchat-prod` branch.
+
+## Upgrade Flow
+
+```bash
+git fetch upstream --tags
+git checkout main
+git merge --ff-only upstream/main
+git push origin main
+
+git checkout andchat-prod
+git merge origin/main
+git push origin andchat-prod
+```
+
+Record every intentional production patch in this file or in a dedicated section below.