`app/gateway/app.py:_check_path_migration_pending` runs at lifespan
startup and emits a single WARNING log when `{base_dir}/users/` still
has content — the operator's cue to run the new migration script. The
warning is best-effort (silent on permission errors) so it can never
escalate into a gateway boot failure; reads from the legacy tree keep
working via the `user_id` branch of `Paths.thread_dir` until the
migration runs.
The root Makefile gains a `migrate-paths` target that wraps
`scripts/migrate_paths_to_workspace.py`. Two opt-in env knobs:
- `DRY_RUN=1` — pass `--dry-run` (preview only, no writes)
- `DEFAULT_WORKSPACE=…` — claim un-assigned users under this workspace id
(defaults to `legacy_workspace` to align with PR5's orphan-row bucket).
`help` documents the new target.
3 tests cover the warning's three states (legacy dir with content, dir
missing, dir empty).
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