fix: remove DefaultServeMux registration and expose webhook route via ConfigFragment

http.HandleFunc on the global DefaultServeMux is never reachable from the
engine because the plugin runs as a separate gRPC process. Removed the
http.HandleFunc call from Start() and implemented sdk.ConfigProvider on
githubPlugin, returning a config fragment that declares the
/webhooks/github HTTP route so the engine's own HTTP server registers it
through the normal config pipeline.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: intel352

internal/module_webhook.go

@@ -100,12 +100,9 @@ func (m *webhookModule) SetMessageSubscriber(_ sdk.MessageSubscriber) {}
 // Init is a no-op; the module is ready after construction.
 func (m *webhookModule) Init() error { return nil }
 
-// Start registers the HTTP webhook handler with the global mux.
-// The engine must be running its HTTP server for this to receive requests.
-func (m *webhookModule) Start(_ context.Context) error {
-	http.HandleFunc("/webhooks/github", m.handleWebhook)
-	return nil
-}
+// Start is a no-op; the webhook route is declared via ConfigFragment so the
+// engine's HTTP server registers it through the normal config pipeline.
+func (m *webhookModule) Start(_ context.Context) error { return nil }
 
 // Stop is a no-op.
 func (m *webhookModule) Stop(_ context.Context) error { return nil }

internal/plugin.go

@@ -6,6 +6,20 @@ import (
 	sdk "github.com/GoCodeAlone/workflow/plugin/external/sdk"
 )
 
+// webhookRouteConfig is the config fragment YAML that declares the GitHub
+// webhook HTTP route so the engine's HTTP server registers it via the normal
+// config pipeline instead of the unreachable global DefaultServeMux.
+const webhookRouteConfig = `
+workflows:
+  github-webhook-receiver:
+    triggers:
+      - type: http
+        config:
+          path: /webhooks/github
+          method: POST
+    steps: []
+`
+
 // githubPlugin implements sdk.PluginProvider, sdk.ModuleProvider, and sdk.StepProvider.
 type githubPlugin struct{}
 
@@ -61,3 +75,11 @@ func (p *githubPlugin) CreateStep(typeName, name string, config map[string]any)
 		return nil, fmt.Errorf("github plugin: unknown step type %q", typeName)
 	}
 }
+
+// ConfigFragment implements sdk.ConfigProvider.
+// It returns a config fragment that declares the /webhooks/github HTTP route
+// so the engine registers it through the normal config pipeline rather than
+// through the global DefaultServeMux (which is unreachable in a gRPC plugin).
+func (p *githubPlugin) ConfigFragment() ([]byte, error) {
+	return []byte(webhookRouteConfig), nil
+}