fix(/review git_diff not considering submodules): Fix review diff to include submodule changes

[guidedways]

Summary

• teach get_git_diff to gather tracked/untracked diffs for each submodule in parallel, so /review uncommitted changes sees edits inside nested repos
• skip .gitmodules entries that are not yet initialized worktrees, preventing /review from failing on freshly cloned repos
• add several regression tests covering both the initialized submodule scenario and the uninitialized placeholder case

Testing

• just fmt
• cargo test -p codex-tui

Fixes #5596