`scripts/migrate_paths_to_workspace.py` walks the PR4 per-user filesystem
layout and rewrites it under the PR6 per-workspace dimension:
- `{base}/users/{uid}/threads/{tid}/` → `{base}/workspaces/{wid}/threads/{tid}/`
- `{base}/users/{uid}/memory.json` → `{base}/workspaces/{wid}/users/{uid}/memory.json`
- `{base}/users/{uid}/agents/{name}/` → `{base}/workspaces/{wid}/users/{uid}/agents/{name}/`
`{wid}` is read from `users.default_workspace_id` in the local sqlite
DB; users without one fall back to `--default-workspace`, defaulting to
`legacy_workspace` (the same bucket PR5's row backfill uses for orphan
rows so the file-system and database alignments stay consistent).
Pre-existing destinations route the legacy copy to
`{base}/migration-conflicts/workspace-migration/...` for manual review;
empty source dirs are cleaned up. The script is idempotent and supports
`--dry-run` (writes nothing, still logs the full report).
9 tests cover: thread / memory / agent rewrite, dry-run no-write, the
fallback workspace path, conflict diversion, post-migration cleanup of
empty `users/` dirs, and the missing-DB / missing-table degraded
modes.
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