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

The http and slack targets sent the constant User-Agent webhooker/1.0. They now send webhooker/ followed by the version the build stamped, the same value the footer shows, built in one place on the delivery engine. User-Agent stays a reserved header and is still set after the target's configured headers, so a configured one cannot override it. Tests check the header each target sends against a known version, and that a configured User-Agent is replaced.

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,