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/lib.rs

@@ -18,9 +18,7 @@ use tracing_subscriber::layer::SubscriberExt;
 use tracing_subscriber::util::SubscriberInitExt;
 use tracing_subscriber::{EnvFilter, Layer};
 
-use lumotia_core::paths::{
-    check_target_ambiguity, migrate_legacy_data_dir, MigrationStatus,
-};
+use lumotia_core::paths::{check_target_ambiguity, migrate_legacy_data_dir, MigrationStatus};
 use lumotia_core::types::EngineName;
 use lumotia_llm::LlmEngine;
 use lumotia_storage::{
@@ -239,7 +237,7 @@ fn build_rolling_appender(logs_dir: &Path) -> std::io::Result<RollingFileAppende
         .filename_suffix("log")
         .max_log_files(7)
         .build(logs_dir)
-        .map_err(|e| std::io::Error::new(std::io::ErrorKind::Other, e))
+        .map_err(std::io::Error::other)
 }
 
 /// Install a `tracing` subscriber that writes to stderr (developer
@@ -278,9 +276,7 @@ pub fn install_subscriber(logs_dir: &Path) -> Option<WorkerGuard> {
             // Best-effort: keep stderr logging even if the file path is
             // unwritable, and surface the failure to stderr so dogfooders
             // notice the missing forensic stream.
-            let _ = tracing_subscriber::registry()
-                .with(stderr_layer)
-                .try_init();
+            let _ = tracing_subscriber::registry().with(stderr_layer).try_init();
             eprintln!(
                 "lumotia: failed to install rolling file log appender at {}: {e}",
                 logs_dir.display()