Remove unnecessary intermediate `String` allocations when constructing `Arc<str>` and `SharedString` values. Use static `SharedString` storage for fixed worktree picker identifiers. Validation: - `cargo fmt --all -- --check` - `tooling/lints/single-lint owned_string_into_shared -p editor -p git_ui_core -p toolchain_selector` - `cargo test -p editor -p git_ui_core -p toolchain_selector` Release Notes: - N/A