The 17.2.9 revert (172ce9b) restored the legacy path-based session
directory names but removed all migration, including the reverse path.
Sessions written under the short-lived hashed scheme (17.2.5-17.2.8,
`<scope>-<readable>-<sha256>`) were left orphaned, so `omp -r`
current-folder scope reported no sessions.
computeDefaultSessionDir now reconstructs the hashed dir name for the
cwd and performs a one-way best-effort migration into the legacy name,
alongside the existing legacy-absolute migration.
Fixes#7677