sketch: add support for 'external' message types

- adds a new CodingAgentMessageType for loop.AgentMessage
- adds an new /external handler to loophttp.go
- modifies Agent to pass the .TextContent of ExternalMessage into the convo
  as though it came from the user.
- adds sketch-external-message web component, with a template for
  github workflow run events specifically.
- adds demos for sketch-external-message
diff --git a/cmd/go2ts/go2ts.go b/cmd/go2ts/go2ts.go
index a4015e0..956751c 100644
--- a/cmd/go2ts/go2ts.go
+++ b/cmd/go2ts/go2ts.go
@@ -50,6 +50,7 @@
 			loop.PortMessageType,
 			loop.CompactMessageType,
 			loop.SlugMessageType,
+			loop.ExternalMessageType,
 		},
 	)