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/go.sum b/go.sum
index 9f65af8..5e1d734 100644
--- a/go.sum
+++ b/go.sum
@@ -42,6 +42,8 @@
 go.skia.org/infra v0.0.0-20250421160028-59e18403fd4a/go.mod h1:itQeLiwIYtXPJJEqdxRpOlS77LNv/quHjkyy+SaXrkw=
 golang.org/x/crypto v0.37.0 h1:kJNSjF/Xp7kU0iB2Z+9viTPMW4EqqsrywMXLJOOsXSE=
 golang.org/x/crypto v0.37.0/go.mod h1:vg+k43peMZ0pUMhYmVAWysMK35e6ioLh3wB8ZCAfbVc=
+github.com/sashabaranov/go-openai v1.38.2 h1:akrssjj+6DY3lWuDwHv6cBvJ8Z+FZDM9XEaaYFt0Auo=
+github.com/sashabaranov/go-openai v1.38.2/go.mod h1:lj5b/K+zjTSFxVLijLSTDZuP7adOgerWeFyZLUhAKRg=
 golang.org/x/mod v0.24.0 h1:ZfthKaKaT4NrhGVZHO1/WDTwGES4De8KtWO0SIbNJMU=
 golang.org/x/mod v0.24.0/go.mod h1:IXM97Txy2VM4PJ3gI61r1YEk/gAj6zAHN3AdZt6S9Ww=
 golang.org/x/net v0.38.0 h1:vRMAPTMaeGqVhG5QyLJHqNDwecKTomGeqbnfZyKlBI8=