Retire legacy gateway-state migration directory locks

## Context

PR #10845 replaces `.gateway-state-migration.lock` directory locks with descriptor-held, provenance-bearing regular-file generations. The new protocol reclaims and releases only an observed file generation, so a concurrent replacement remains canonical.

For direct upgrades, the migration path still recognizes the old directory shape as contention. Because its PID-only `owner` file does not prove which host or PID namespace created it, NemoClaw never reclaims the directory automatically, even when that PID is absent locally. Recovery requires operator resolution.

## Retirement exit criteria

Remove the legacy directory compatibility branch and its focused tests once the documented minimum supported direct-upgrade source release is at or after the first release containing #10845. At that point every supported source release uses the provenance-bearing regular-file migration lock format, so no supported version can create the legacy directory.

If NemoClaw does not yet publish a minimum supported direct-upgrade source version, keep this issue open until that policy and cutoff are documented.

First verify whether NemoClaw documents a minimum supported direct-upgrade source release and whether it is at or after the release containing PR #10845. Then locate the direct-upgrade migration path and its focused tests; done means the legacy directory compatibility branch and tests are removed only after that cutoff is documented.

Written by the indexing model from the issue text.