fix(buf): generate C# service clients from aggregate template

[mikemcdougall]

Summary

Two coupled C# code-generation defects in the canonical templates/docs, found in the round-5 release audit.

1. Aggregate buf.gen.yaml omits the grpc/csharp plugin (S2)

buf.gen.yaml:16 — the aggregate C# block listed only protocolbuffers/csharp and was missing grpc/csharp, even though its own comment asserts it "matches buf.gen.csharp.yaml and the published NuGet package." Every other language pairs its protobuf plugin with its gRPC plugin (Go, Java, Python); C# was the lone gap.

Consequence: buf generate — the exact command docs/getting-started.md Step 3 tells C# users to run — emitted message/enum types but no *ServiceClient stubs. The Step 5 sample (new FeatureService.FeatureServiceClient(channel) + QueryFeaturesAsync) would not compile against that output. The NuGet package is unaffected (it uses Grpc.Tools with GrpcServices=Both), so only the documented local-codegen C# path was broken, and CI's per-language generate job uses buf.gen.csharp.yaml — so this landed green.

Fix: add grpc/csharp:v1.81.1 (pinned to match buf.gen.csharp.yaml) to the aggregate template.

2. AGENTS.md documents a wrong/breaking C# base namespace (S2)

AGENTS.md:115 claimed generation uses "C# base namespace GeospatialGrpc". No base_namespace override is — or can be — set: managed mode derives Geospatial.V1 from the proto package, and a base_namespace that is not a prefix of that namespace makes the csharp plugin hard-fail. All real consumers (examples/dotnet/Program.cs, getting-started, the smoke test) use Geospatial.V1. A contributor trusting AGENTS.md and adding base_namespace=GeospatialGrpc would break C# generation. Corrected the text to match buf.gen.yaml's own comment.

Verification

Ran buf generate with the fixed aggregate C# block:

• Output namespace: Geospatial.V1 (matches all consumers)
• 14 *ServiceClient classes generated (FeatureServiceClient, FormServiceClient, etc.)

Empirically confirmed the inverse: setting base_namespace=GeospatialGrpc fails with Namespace Geospatial.V1 is not a prefix namespace of base namespace GeospatialGrpc.

Findings addressed

• buf.gen.yaml:16 — aggregate template missing grpc/csharp plugin
• AGENTS.md:115 — stale/incorrect C# base-namespace claim

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