Add experimental API annotations from schema stability markers

Read the 'stability' field from api.schema.json and propagate it to
generated code in all four SDK languages. APIs marked as experimental
in the schema (fleet, agent, compaction) now carry appropriate
annotations in the generated output.

Changes to codegen scripts (scripts/codegen/):
- utils.ts: Add stability field to RpcMethod; add isNodeFullyExperimental helper
- csharp.ts: Emit [Experimental(Diagnostics.Experimental)] on types and API classes
- typescript.ts: Emit /** @experimental */ JSDoc on types and groups
- python.ts: Emit # Experimental comments on types and API classes, docstrings on methods
- go.ts: Emit // Experimental: comments on types and API structs

Design decisions:
- When all methods in a group are experimental, the group/class is
  annotated and individual methods are not (annotation is inherited)
- Data types (request/result) for experimental methods are also annotated
- C# uses a Diagnostics.Experimental const ("GHCP001") referenced by all attributes
- SDK csproj suppresses GHCP001 internally; consumers still see warnings

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

dotnet/src/Generated/Rpc.cs

@@ -5,12 +5,20 @@
 // AUTO-GENERATED FILE - DO NOT EDIT
 // Generated from: api.schema.json
 
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 using StreamJsonRpc;
 
 namespace GitHub.Copilot.SDK.Rpc;
 
+/// <summary>Diagnostic IDs for the Copilot SDK.</summary>
+internal static class Diagnostics
+{
+    /// <summary>Indicates an experimental API that may change or be removed.</summary>
+    internal const string Experimental = "GHCP001";
+}
+
 /// <summary>RPC data type for Ping operations.</summary>
 public class PingResult
 {
@@ -427,6 +435,7 @@ internal class SessionWorkspaceCreateFileRequest
 }
 
 /// <summary>RPC data type for SessionFleetStart operations.</summary>
+[Experimental(Diagnostics.Experimental)]
 public class SessionFleetStartResult
 {
     /// <summary>Whether fleet mode was successfully activated.</summary>
@@ -435,6 +444,7 @@ public class SessionFleetStartResult
 }
 
 /// <summary>RPC data type for SessionFleetStart operations.</summary>
+[Experimental(Diagnostics.Experimental)]
 internal class SessionFleetStartRequest
 {
     /// <summary>Target session identifier.</summary>
@@ -463,6 +473,7 @@ public class Agent
 }
 
 /// <summary>RPC data type for SessionAgentList operations.</summary>
+[Experimental(Diagnostics.Experimental)]
 public class SessionAgentListResult
 {
     /// <summary>Available custom agents.</summary>
@@ -471,6 +482,7 @@ public class SessionAgentListResult
 }
 
 /// <summary>RPC data type for SessionAgentList operations.</summary>
+[Experimental(Diagnostics.Experimental)]
 internal class SessionAgentListRequest
 {
     /// <summary>Target session identifier.</summary>
@@ -495,6 +507,7 @@ public class SessionAgentGetCurrentResultAgent
 }
 
 /// <summary>RPC data type for SessionAgentGetCurrent operations.</summary>
+[Experimental(Diagnostics.Experimental)]
 public class SessionAgentGetCurrentResult
 {
     /// <summary>Currently selected custom agent, or null if using the default agent.</summary>
@@ -503,6 +516,7 @@ public class SessionAgentGetCurrentResult
 }
 
 /// <summary>RPC data type for SessionAgentGetCurrent operations.</summary>
+[Experimental(Diagnostics.Experimental)]
 internal class SessionAgentGetCurrentRequest
 {
     /// <summary>Target session identifier.</summary>
@@ -527,6 +541,7 @@ public class SessionAgentSelectResultAgent
 }
 
 /// <summary>RPC data type for SessionAgentSelect operations.</summary>
+[Experimental(Diagnostics.Experimental)]
 public class SessionAgentSelectResult
 {
     /// <summary>The newly selected custom agent.</summary>
@@ -535,6 +550,7 @@ public class SessionAgentSelectResult
 }
 
 /// <summary>RPC data type for SessionAgentSelect operations.</summary>
+[Experimental(Diagnostics.Experimental)]
 internal class SessionAgentSelectRequest
 {
     /// <summary>Target session identifier.</summary>
@@ -547,11 +563,13 @@ internal class SessionAgentSelectRequest
 }
 
 /// <summary>RPC data type for SessionAgentDeselect operations.</summary>
+[Experimental(Diagnostics.Experimental)]
 public class SessionAgentDeselectResult
 {
 }
 
 /// <summary>RPC data type for SessionAgentDeselect operations.</summary>
+[Experimental(Diagnostics.Experimental)]
 internal class SessionAgentDeselectRequest
 {
     /// <summary>Target session identifier.</summary>
@@ -560,6 +578,7 @@ internal class SessionAgentDeselectRequest
 }
 
 /// <summary>RPC data type for SessionCompactionCompact operations.</summary>
+[Experimental(Diagnostics.Experimental)]
 public class SessionCompactionCompactResult
 {
     /// <summary>Whether compaction completed successfully.</summary>
@@ -576,6 +595,7 @@ public class SessionCompactionCompactResult
 }
 
 /// <summary>RPC data type for SessionCompactionCompact operations.</summary>
+[Experimental(Diagnostics.Experimental)]
 internal class SessionCompactionCompactRequest
 {
     /// <summary>Target session identifier.</summary>
@@ -1000,6 +1020,7 @@ public async Task<SessionWorkspaceCreateFileResult> CreateFileAsync(string path,
 }
 
 /// <summary>Provides session-scoped Fleet APIs.</summary>
+[Experimental(Diagnostics.Experimental)]
 public class FleetApi
 {
     private readonly JsonRpc _rpc;
@@ -1020,6 +1041,7 @@ public async Task<SessionFleetStartResult> StartAsync(string? prompt = null, Can
 }
 
 /// <summary>Provides session-scoped Agent APIs.</summary>
+[Experimental(Diagnostics.Experimental)]
 public class AgentApi
 {
     private readonly JsonRpc _rpc;
@@ -1061,6 +1083,7 @@ public async Task<SessionAgentDeselectResult> DeselectAsync(CancellationToken ca
 }
 
 /// <summary>Provides session-scoped Compaction APIs.</summary>
+[Experimental(Diagnostics.Experimental)]
 public class CompactionApi
 {
     private readonly JsonRpc _rpc;

dotnet/src/GitHub.Copilot.SDK.csproj

@@ -20,6 +20,10 @@
         <EmbedUntrackedSources>true</EmbedUntrackedSources>
     </PropertyGroup>
 
+  <PropertyGroup>
+    <NoWarn>$(NoWarn);GHCP001</NoWarn>
+  </PropertyGroup>
+
     <PropertyGroup Condition="'$(CI)' == 'true' or '$(TF_BUILD)' == 'true'">
         <ContinuousIntegrationBuild>true</ContinuousIntegrationBuild>
     </PropertyGroup>

dotnet/test/GitHub.Copilot.SDK.Test.csproj

@@ -2,6 +2,7 @@
 
   <PropertyGroup>
     <IsPackable>false</IsPackable>
+    <NoWarn>$(NoWarn);GHCP001</NoWarn>
   </PropertyGroup>
 
   <PropertyGroup>