fix/batch: prevent host execution through Git config

[cbrnrd]

Problem

Batch steps run repository code inside a Docker container, but bind workspaces also expose the repository's .git directory to that container. A step can change .git/config and add an external diff command or Git filter. After the step finishes, src-cli runs git add and git diff on the host. Those commands can read the changed configuration and run the repository-provided command outside the container with the permissions of src-cli.

Solution

Save the trusted Git control files before repository code runs and restore them before host-side Git operations. This prevents a container step from changing the configuration used by host Git. The diff command also uses --no-ext-diff as an additional safeguard. The restore supports normal repositories and linked worktrees, and rejects unsafe Git metadata replacements.

This differs from the recent archive and Git hook fixes. Those changes reject Git metadata supplied during workspace creation and disable hooks. This change protects Git configuration modified later by a running container step, including external diff commands and clean filters that do not use hooks.

Verification Evidence

• Added a regression test that injects an external diff command and a clean filter into .git/config. The diff succeeds without running either command, and the trusted config is restored.
• go test ./...
• go vet ./internal/batches/workspace ./internal/batches/executor
• Windows amd64 cross-compilation of the workspace tests