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/app.css

@@ -4160,44 +4160,6 @@
     color: #f5f7ff;
     background: linear-gradient(180deg, rgba(100,108,255,0.22), rgba(65,209,255,0.1));
   }
-  .ai-provider-option-local {
-    flex-wrap: wrap;
-    row-gap: 2px;
-  }
-  .ai-provider-badge {
-    flex-basis: 100%;
-    text-align: center;
-    font-size: 9.5px;
-    font-weight: 700;
-    letter-spacing: 0.04em;
-    text-transform: uppercase;
-    color: var(--color-ink-faint);
-  }
-  .ai-local-profile-options {
-    display: grid;
-    grid-template-columns: repeat(3, minmax(0, 1fr));
-    gap: 8px;
-  }
-  .ai-local-profile-option {
-    min-width: 0;
-    min-height: 34px;
-    padding: 0 8px;
-    border-color: var(--color-border-subtle);
-    background: rgba(255,255,255,0.03);
-    color: var(--color-ink-dim);
-    font-size: 12px;
-    font-weight: 700;
-  }
-  .ai-local-profile-option:hover:not(:disabled) {
-    border-color: var(--color-border);
-    color: var(--color-ink);
-    background: var(--color-surface-hover);
-  }
-  .ai-local-profile-option.active {
-    border-color: rgba(65,209,255,0.48);
-    color: #f5f7ff;
-    background: linear-gradient(180deg, rgba(65,209,255,0.16), rgba(100,108,255,0.12));
-  }
   .new-branch-form {
     display: flex;
     flex-direction: column;