chore: bootstrap center/guild backend workspaces

Fabric.Backend.Center/README.md

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+# Fabric.Backend.Center
+
+Identity Hub service for Fabric.
+
+## Scope (MVP)
+- User register/login
+- Session/token management
+- Guild Node registration + shared-secret handshake
+- Center-level audit logs
+
+## Next
+- API skeleton (NestJS)
+- Auth module
+- Guild node registry module

Fabric.Backend.Guild/README.md

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+# Fabric.Backend.Guild
+
+Guild Node service for Fabric.
+
+## Scope (MVP)
+- Workspace/Guild/Channel/DM
+- Message create/edit/delete/reply/@mention
+- Per-channel/DM seq ordering + gap backfill API
+- Webhook/Bot integration surface
+- Guild-level audit logs
+
+## Next
+- API skeleton (NestJS)
+- Chat domain models
+- Seq allocator and range query endpoints