chore(secrets): deprecate Replicator.Grpc — out of the build and pack (scadaproj#8)

Owner chose option (b). ZB.MOM.WW.Secrets.Replicator.Grpc has had no consumer
since ScadaBridge retired the gRPC secrets hub (2026-09-28); OtOpcUa never used it.
- removed the project and its test project from ZB.MOM.WW.Secrets.slnx, so the
  family build/test and build/push.sh's `dotnet pack` skip it;
- csproj IsPackable=false (a direct pack of the project yields nothing);
- SecretsGrpcHubServerExtensions / SecretsGrpcHubClientExtensions [Obsolete];
- README, CHANGELOG (0.6.6 Deprecated), CLAUDE.md and clustered-secrets.md say so.
Source + tests stay in the tree and still build/run standalone (test csproj
suppresses CS0618): 200/200. Feed versions 0.4.1-0.6.3 are left for the owner
to unlist.

Also zeroes the solution's pre-existing warnings: CA1416 on two Unix-only
permission tests (guarded by Skip.IfNot, which the analyzer cannot see — now
[UnsupportedOSPlatform("windows")]) and two xUnit2031 Assert.Single(Where).
Build 0 warnings; tests 429 passed / 21 skipped / 0 failed; pack = 5 nupkgs.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

ZB.MOM.WW.Secrets/docs/operations/clustered-secrets.md

@@ -221,7 +221,11 @@ Which topology each app runs in production is decided — tracked as
   propagation proven with seeded isolation controls on the other pairs** rather than by mere
   absence. It supersedes the 2026-07-18 six-node gate as evidence, which predates the mesh
   Phase 6 partition.
-- **ScadaBridge: central gRPC hub (§2's SQL-hub mode is NOT used).** Site servers never talk to
+- **ScadaBridge: central gRPC hub (§2's SQL-hub mode is NOT used).** *Historical — superseded
+  2026-09-28: ScadaBridge retired the gRPC hub (site secrets ride its artifact deploy), and the
+  `ZB.MOM.WW.Secrets.Replicator.Grpc` package is **deprecated** as of family `0.6.6`
+  (scadaproj#8: no longer built or packed; `0.6.3` is its last published version). The record below
+  is kept for the gates it describes.* Site servers never talk to
   the DB directly — they talk to central for everything — so the SqlServer hub's site-side
   connection string violates the topology rule. The `ZB.MOM.WW.Secrets.Replicator.Grpc` package
   (central-hosted `secrets_hub.v1`, **pull-only: updates originate at central only, by design** —