feat(harness): relocate workspaces to user home

[JnyRoad]

Summary

Move the default Harness workspace root from the application directory to ~/.staffdeck/workspaces while keeping database, logs, runtime configuration, and uploads unchanged.

Changes

• Persist each task's effective Harness workspace root for stable artifact access and cleanup.
• Preserve bounded compatibility with configured and legacy workspace roots.
• Validate artifact reads and downloads against task-scoped metadata.
• Present the setting as a Harness-only workspace in Runtime Settings.

Testing

• PYTHONPATH=/private/tmp/staffdeck-pr13-upstream-20260830/backend /Users/lvtu/source/tools/StaffDeck/backend/.venv/bin/python -m pytest backend/tests/test_harness_session_cleanup.py backend/tests/test_ui_config.py backend/tests/test_harness_artifact_download.py backend/tests/test_harness_filesystem.py backend/tests/test_harness_v2.py backend/tests/test_desktop_launcher.py (188 passed)
• npm --prefix frontend-enterprise test -- src/pages/RuntimeSettingsPage.test.tsx (5 passed)
• npm --prefix frontend-enterprise run build (passed)

Risks

The database migration adds a nullable workspace_root field. Existing task frames retain bounded fallback access to configured and legacy roots. The conflict resolution intentionally excludes unrelated fork-only network settings UI and tests.