run_reach raises UnicodeEncodeError for a checkout whose directory name has a non-UTF-8 byte #754

run_reach in dokimi/dokimi/repos/reach.py:527 derives its default salt with hashlib.sha256(f"{tree.label}\0{datetime.now(timezone.utc).isoformat()}".encode()). WorkTree.label is str(self.root) (repos/source.py:122), and a git tree's label starts with the source location, so a checkout whose directory name holds a non-UTF-8 byte (a surrogate-escaped path from argv) raises when no --salt is given. Repro: WorkTree over a directory named d\xff holding the toy repo of tests/test_repo_reach.py, then run_reach(tree=tree, manifest=load_manifest(tree, 'evals/repo-eval.yaml'), profile=..., model='m', out_dir=..., work_dir=..., dry_run=True) gives UnicodeEncodeError: 'utf-8' codec can't encode character '\udcff' in position N: surrogates not allowed at reach.py:527. Fix: os.fsencode(f"..."), which keeps the salt of a UTF-8 label unchanged on POSIX (the label is a path or a source location, so n\ud800 cannot occur). Same class as #715, which fixes token in the same file.

Found while fixing #715. The fix's PR is #761.