feat(plugins): add protocol versioning to plugin interface

Add protocol_version field to Initialize RPC for forward compatibility as the plugin protocol evolves.

Changes:
- Add protocol_version to InitializeRequest (server → plugin)
- Add protocol_version to InitializeResponse (plugin → server)
- Set current protocol version to 1
- Version 0 indicates pre-versioning implementations (treated as v1)

This allows the server and plugins to negotiate capabilities:
- Server can avoid calling RPCs that older plugins don't implement
- Plugins can detect newer servers and enable advanced features
- Graceful degradation when versions mismatch

The AI service now reports protocol version 1 during initialization.

protos/plugin.proto

@@ -28,12 +28,21 @@ service PluginService {
 message InitializeRequest {
     string server_version = 1;
     map<string, string> config = 2;
+    // protocol_version is the plugin protocol version the server supports.
+    // The current version is 1. Plugins should check this to know what RPCs
+    // the server may call. A value of 0 means the server predates versioning.
+    int32 protocol_version = 3;
 }
 
 message InitializeResponse {
     bool success = 1;
     string error = 2;
     PluginManifest manifest = 3;
+    // protocol_version is the plugin protocol version the plugin supports.
+    // The current version is 1. The server uses this to avoid calling RPCs
+    // that the plugin doesn't implement. A value of 0 means the plugin
+    // predates versioning and is treated as protocol version 1.
+    int32 protocol_version = 4;
 }
 
 message ShutdownRequest {

src/GitCaddy.AI.Service/Services/PluginServiceImpl.cs

@@ -13,6 +13,12 @@ namespace GitCaddy.AI.Service.Services;
 /// </summary>
 public class PluginServiceImpl : PluginService.PluginServiceBase
 {
+    /// <summary>
+    /// The plugin protocol version this implementation supports.
+    /// Increment when new RPCs are added to PluginService.
+    /// </summary>
+    private const int ProtocolVersion = 1;
+
     private readonly ILogger<PluginServiceImpl> _logger;
     private readonly ILicenseValidator _licenseValidator;
 
@@ -27,12 +33,15 @@ public class PluginServiceImpl : PluginService.PluginServiceBase
     public override Task<InitializeResponse> Initialize(
         InitializeRequest request, ServerCallContext context)
     {
-        _logger.LogInformation("Plugin initialized by server version {ServerVersion}", request.ServerVersion);
+        _logger.LogInformation(
+            "Plugin initialized by server version {ServerVersion} (protocol v{ProtocolVersion})",
+            request.ServerVersion, request.ProtocolVersion);
 
         return Task.FromResult(new InitializeResponse
         {
             Success = true,
-            Manifest = BuildManifest()
+            Manifest = BuildManifest(),
+            ProtocolVersion = ProtocolVersion
         });
     }