Invalid ancestor .git marker prevents adding a plain folder

### What happened

A plain folder nested below an incomplete `.git` directory is treated as if it were inside a Git repository. Adding the folder through Project Catalog then invokes Git; `git rev-parse` fails and the mutation surfaces as `project.catalog.mutate → commit_outcome_unknown`.

Expected: if the selected folder has no `.git` entry and an ancestor's `.git` metadata is structurally invalid, Maka should handle the selection like an ordinary non-Git folder. A malformed `.git` entry in the selected folder itself should continue to fail closed.

### How to reproduce

1. Create an incomplete ancestor Git marker and a nested plain folder:
`mkdir -p /tmp/maka-invalid-git/.git/gk /tmp/maka-invalid-git/project`
2. Open Maka Desktop.
3. Add `/tmp/maka-invalid-git/project` as a project.
4. Observe that registration fails with `project.catalog.mutate → commit_outcome_unknown`.

`git -C /tmp/maka-invalid-git/project rev-parse --show-toplevel` also exits 128 because Git discovers the malformed ancestor marker.

### Environment

- Maka version or commit: fde5fb078
- OS and version: macOS 14.7.8
- Surface: Desktop / Runtime Host Project Catalog
- Node.js version, if running from source: v25.9.0

### Logs, screenshots, or additional context

The ancestor `.git` contains only `.git/gk/`; it has no `HEAD`. `hasEnclosingGitEntry` currently treats any lstat-able `.git` entry as a repository marker, so the downstream Git probe receives corrupted ancestor state.