docs: mark snapx as deprecated

Author: peters

README.md

@@ -11,6 +11,11 @@
 
 **snapx** is a powerful xplat .NET application with built-in support for delta updates, release channels (test, staging, production) and automatic deployment using GitHub Actions. Updates can delivered via NuGet or network share (UNC).
 
+> [!WARNING]
+> Snapx is now considered deprecated and should be treated as maintenance-only.
+> New projects should prefer [Surge](https://github.com/fintermobilityas/surge), and existing Snapx users should consider planning a migration to Surge over time.
+> New feature investment is expected to happen in Surge rather than Snapx.
+
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