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-tauri/Cargo.toml

@@ -20,7 +20,7 @@ tauri-plugin-dialog = "=2.7.0"
 tauri-plugin-aptabase = "1.0"
 keyring = { version = "3", features = ["apple-native", "windows-native", "async-secret-service", "crypto-rust", "async-io"] }
 commit_ai = { path = "crates/commit_ai" }
-tokio = "1.52.3"
+tokio = { version = "1.52.3", features = ["macros", "rt-multi-thread"] }
 log = "0.4"
 reqwest = { version = "0.12", default-features = false, features = ["blocking", "json", "rustls-tls"] }
 sysinfo = { version = "=0.38.3", default-features = false, features = ["system"] }