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/redirect_test.go

@@ -375,6 +375,7 @@ func TestApplyRequestHeaders_ReportsOriginScopedNames(t *testing.T) {
 				"Content-Type":  testContentType,
 			},
 		},
+		"webhooker/dev",
 	)
 
 	assert.Equal(t,