refactor(v2/ollama): drop openaicompat shim, use native provider

The Ollama provider now targets /api/chat directly via the native provider
introduced in the previous commits. Public API is unchanged for callers
that go through llm.Ollama() (and is extended by Task 5's OllamaCloud()
constructor).

DefaultBaseURL was renamed to DefaultLocalBaseURL (without the trailing
/v1 segment used by the OpenAI-compat path). registry.go is updated
correspondingly; no other callers referenced the old name.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

v2/registry.go

@@ -129,7 +129,7 @@ var providerRegistry = []ProviderInfo{
 		Name:        "ollama",
 		DisplayName: "Ollama (local)",
 		EnvKey:      "", // no key needed
-		DefaultURL:  ollama.DefaultBaseURL,
+		DefaultURL:  ollama.DefaultLocalBaseURL,
 		Models: []string{
 			"llama3.2", "llama3.1", "qwen2.5", "mistral", "gemma2", "phi4",
 		},