Send the build version in the outbound User-Agent (closes #313)

The http and slack targets sent the fixed User-Agent webhooker/1.0. The
delivery engine now takes the version from Globals, the value the web UI
footer shows, and one method builds webhooker/<version> for both
targets. The http target still sets it after the configured headers, so
a configured or inbound User-Agent never reaches the wire.

A test builds the engine through its real constructor with a known
version and checks the header each target sends.

Model: opus-5-5

internal/delivery/engine_test.go

@@ -1247,11 +1247,6 @@ func TestDoHTTPRequest_ForwardsHeaders(t *testing.T) {
 		testContentType,
 		receivedHeaders.Get("Content-Type"),
 	)
-
-	assert.Equal(t,
-		"webhooker/1.0",
-		receivedHeaders.Get("User-Agent"),
-	)
 }
 
 // The event's stored inbound headers carry the same Content-Type the
@@ -1320,6 +1315,7 @@ func TestApplyRequestHeaders_SendsOneContentType(t *testing.T) {
 					ContentType: tc.event,
 				},
 				cfg,
+				"webhooker/dev",
 			)
 
 			assert.Equal(t,