fix(core): preserve post-push repo context for git -C push

[skulidropek]

Closes #201

Summary

• resolve the effective repo root from the original git invocation before running the post-push action
• forward that repo root via DOCKER_GIT_POST_PUSH_REPO_ROOT so session backup runs in the pushed repository instead of the caller cwd
• keep the existing invariant that post-push backup runs only after successful non-dry-run pushes
• add regression coverage for normal git push, git -C <repo> push, and dry-run push variants

Verification

• pnpm install --frozen-lockfile
• pnpm --filter ./packages/lib typecheck
• pnpm --filter ./packages/lib test -- tests/core/templates.test.ts tests/core/git-post-push-wrapper.test.ts