fix(core): restore post-push backup via git wrapper

[skulidropek]

Closes #198.

Summary

• replace the non-functional client-side post-push hook assumption with a managed git wrapper in the generated container
• keep pre-push protections in hooks while running the session backup post-action only after a successful real git push
• skip post-actions for git push --dry-run and guard internal backup pushes from recursion
• update template expectations and post-action wording to match the runtime behavior

Root Cause

Git does not execute a client-side post-push hook, so the previously generated hook file under core.hooksPath was never called.

Validation

• pnpm --filter ./packages/lib typecheck
• pnpm --filter ./packages/lib test
• pnpm --filter ./packages/app exec vitest run tests/docker-git/entrypoint-auth.test.ts