all: support openai-compatible models

The support is rather minimal at this point:
Only hard-coded models, only -unsafe, only -skabandaddr="".

The "shared" LLM package is strongly Claude-flavored.

We can fix all of this and more over time, if we are inspired to.
(Maybe we'll switch to https://github.com/maruel/genai?)

The goal for now is to get the rough structure in place.
I've rebased and rebuilt this more times than I care to remember.
diff --git a/cmd/go2ts/go2ts.go b/cmd/go2ts/go2ts.go
index f7d773f..0a175a9 100644
--- a/cmd/go2ts/go2ts.go
+++ b/cmd/go2ts/go2ts.go
@@ -12,7 +12,7 @@
 	"os"
 
 	"go.skia.org/infra/go/go2ts"
-	"sketch.dev/ant"
+	"sketch.dev/llm"
 	"sketch.dev/loop"
 	"sketch.dev/loop/server"
 )
@@ -54,7 +54,7 @@
 		loop.AgentMessage{},
 		loop.GitCommit{},
 		loop.ToolCall{},
-		ant.Usage{},
+		llm.Usage{},
 		server.State{},
 	)