fix(service): add workspace root to permission context

[Premsenareddy]

Summary

• add a workspace-visible root path interface to WorkspaceBase and concrete workspace backends
• include the resolved workspace root in PermissionContext.working_directories during ChatService agent assembly
• preserve existing working directory entries when already configured

Fixes #1791

Testing

• .venv/bin/python -m pytest -p no:cacheprovider tests/chat_service_test.py tests/service_toolkit_test.py tests/permission_engine_test.py tests/permission_mode_test.py -q
• .venv/bin/python -m pytest -p no:cacheprovider tests/builtin_read_test.py tests/builtin_write_test.py tests/builtin_glob_test.py tests/builtin_grep_test.py tests/workspace_local_test.py tests/builtin_bash_test.py -q
• git diff --check
• py_compile on touched files