feat(submodules): add submodule management and recursive clone

Add a new backend module to manage Git submodules (list, initialize/update,
stage, sync, add) and expose Tauri commands (list_submodules,
submodule_action, add_submodule). The implementation enforces safe relative
paths, a maximum nesting depth, and guards (dirty/conflicted checks and
initialization/no-op semantics) to avoid unsafe operations.

Refactor clone logic to a testable run_git_clone_command and enable
automatic initialization of submodules during clone by passing
--recurse-submodules. Also disallow certain submodule-related custom clone
flags so callers cannot override this behaviour.

Update README with a Submodules section and add frontend components and types
to surface submodule UI (dialogs, toolbar badge). Unit tests were added for
submodule discovery, initialization/update semantics, and recursive clone
behavior.

src-tauri/src/main.rs

@@ -10,6 +10,7 @@ use badge::set_sync_badge;
 use external_tools::{
     detect_external_tools, launch_external_diff, launch_external_merge, launch_external_tool,
 };
+use git::submodules::{add_submodule, list_submodules, submodule_action};
 use git::{
     SearchCancellationState, add_remote, add_to_gitignore, add_worktree, amend_commit,
     apply_file_patch, cancel_code_search, cancel_file_history, checkout_branch, cherry_pick_abort,
@@ -362,6 +363,9 @@ async fn main() {
             rename_branch,
             rename_remote_branch,
             delete_branch,
+            list_submodules,
+            add_submodule,
+            submodule_action,
             list_worktrees,
             add_worktree,
             remove_worktree,