agent: lumotia — pin rust toolchain + workspace clippy/fmt sweep

rust-toolchain.toml pins to stable 1.94.1 so contributors and CI runners
share the exact rustc / rustfmt / clippy versions. Without the pin, every
machine surfaces a different lint set depending on its local install — six
pre-existing lints showed up on 1.94.1 that 1.93-era HANDOVER reported clean.

Clippy fixes (all pre-existing, not introduced by feature work):

- crates/storage/src/database.rs: std::iter::repeat().take() -> repeat_n()
- crates/llm/src/lib.rs (docs): "+ frontends" was parsed as a markdown bullet
  continuation by rustdoc, breaking doc-lazy-continuation. Reworded to "and".
- crates/llm/src/lib.rs (loop): while-let-on-iterator -> for-loop.
- src-tauri/src/commands/security.rs: .iter().any(|a| *a == x) -> .contains(&x).
- src-tauri/src/lib.rs: io::Error::new(Other, e) -> io::Error::other(e).
- src-tauri/src/tauri_app_data_migration.rs: drop function-tail `return`s
  inside cfg blocks; each platform's block now ends with a tail expression.

cargo fmt sweep across the workspace. Mechanical layout-only changes;
no semantics affected.

Workspace gates after this commit:
- cargo fmt --check: clean
- cargo clippy --workspace --all-targets -- -D warnings: clean
- cargo test --workspace: 405/0 (will become 409/0 with Phase A.1+A.2)

src-tauri/src/commands/fs.rs

@@ -88,9 +88,12 @@ pub(crate) fn resolve_export_path(path: &Path, bases: &[PathBuf]) -> Result<Path
             path.display()
         )
     })?;
-    let file_name = path
-        .file_name()
-        .ok_or_else(|| format!("Refusing to write {}: path has no filename.", path.display()))?;
+    let file_name = path.file_name().ok_or_else(|| {
+        format!(
+            "Refusing to write {}: path has no filename.",
+            path.display()
+        )
+    })?;
 
     let canon_parent = std::fs::canonicalize(parent).map_err(|e| {
         format!(