The shell safety guard in _guard_command() checked extracted absolute
paths only against cwd_path (the command's working directory). When
cwd was a subdirectory like ~/.nanobot/workspace/obsidian_notes, any
absolute path referencing the broader workspace (e.g. a sibling
directory or the workspace root itself) was incorrectly blocked with
"path outside working dir".
Fix: pass the workspace_root from _prepare_command() into
_guard_command() and check absolute paths against it as a fallback
when they are outside cwd_path. Paths truly outside the workspace
root are still blocked.
Added tests:
- test_exec_allows_workspace_paths_from_subdirectory
- test_exec_blocks_outside_paths_from_subdirectory