[P1] Full-rebuild ownership check can allow deletion of the source project #9

With build_dir: ".", an ordinary build creates .redun-cache.db beside the config—even if the build then fails because the target is missing. A subsequent --full-rebuild treats that database as proof that the directory is disposable and recursively deletes the config, source files, and everything else in it.

Relevant code: main.py, lines 37–44 and database creation during build, lines 117–120.

Reproduction in an isolated temporary directory:

1. Put a valid config and a source file in the directory; set build_dir: ".".
2. Run a normal build with the configured target absent. The build fails, but leaves .redun-cache.db.
3. Run with --full-rebuild. The ownership check now passes and deletes the directory.

The review probe confirmed that even the failed first build created the marker, and that the cleanup subsequently removed both the original config and source.

Expected behavior: reject unsafe build-directory ownership before creating the cache marker, or otherwise ensure cleanup cannot remove source/config/unrelated files. The presence of a database alone is insufficient evidence of ownership.

Found while reviewing PR #7, toolchain at ce12c48.