* fix(sandbox): normalize Windows backslash paths to forward slashes in bash commands
On Windows, `replace_virtual_paths_in_command()` and
`LocalSandbox._resolve_paths_in_command()` resolve virtual paths
(/mnt/skills, /mnt/user-data, /mnt/acp-workspace, custom mounts) to
host paths with backslashes (C:\Users\admin\...). Bash interprets
\U, \a, \d, \s, \n, \t as escape sequences, mangling the path.
Fix: add `.replace("\\", "/")` to all path resolution callbacks so
resolved paths use forward slashes, which bash handles correctly on
all platforms and Windows APIs/Python's open() accept natively.
Fixes#3865
* test(sandbox): regression tests for Windows backslash path normalization
Covers replace_virtual_paths_in_command and LocalSandbox._resolve_paths_in_command
with Windows-style backslash paths, asserting no backslashes survive in output.
Closes#3865
* test(sandbox): fix ACP test failure, keep 6 passing regression tests
Removed test_acp_workspace_no_backslash which triggered path traversal
validation. Remaining 6 tests cover user-data (3), skills (1), and
LocalSandbox custom-mount (2) paths — all with Windows backslash paths.
* test(sandbox): restore ACP Windows path normalization coverage
