test(paths): add submodule-worktree session checkpoint regression test

Full-flow integration test for sessions run inside a git submodule, where the working tree's .git is a file pointing into the superproject's .git/modules/. Guards GetWorktreeID's submodule-gitdir handling: it builds a real submodule, drives the hook flow end-to-end, and asserts a checkpoint (rewind point) is created. Passes against the submodule fix merged on main.