refactor(commit-ai): remove local models and simplify AI flow

Remove local on-device model support and related IPC commands,
consolidating commit-generation to cloud providers and simplifying the
AI crate surface. Local-specific types, generation profiles, caching,
and the local prompt builder were removed while message sanitization and
diff-echo detection were preserved. Also harden repository handling and
runtime: unborn HEADs are handled gracefully so empty repos still report
files, Git LFS sync is skipped for repositories without commits, and
tokio runtime features were enabled.

- Remove local model engine, load/status commands, and local profile code
- Handle unborn HEAD and skip LFS sync for repos without commits
- Enable tokio runtime features and route AI generation to cloud only

src/lib/components/AiReviewDialog.svelte

@@ -15,8 +15,7 @@
   function providerLabel(value: CommitAiProvider): string {
     if (value === "openai") return "OpenAI";
     if (value === "anthropic") return "Anthropic";
-    if (value === "custom") return "Custom endpoint";
-    return "Local AI";
+    return "Custom endpoint";
   }
 
   function locationLabel(finding: AiReviewFinding): string {