Fix auth snapshots to use filesystem paths (#1296)

Fix auth snapshots to use filesystem paths (#1296)

What changed:
- `auth:snapshot` and `auth:restore` now resolve real filesystem paths instead of Laravel `local` disk paths.
- Missing parent directories are created with restrictive permissions, and snapshot files are written with `0600`.
- Default snapshot location is now the OS temp directory under `inventory-auth-snapshots`.
- Added shared path resolution in AuthSnapshotFile.php.
- Updated tests in AuthSnapshotCommandTest.php to use real temp files and cover absolute paths, parent directory creation, and relative paths.
- Updated the full import runbooks in legacy-importer.agent.md, including OVH using `/tmp/inventory-auth-snapshots/pre-import.json.enc`.

Verification:
- Console suite passed in the project container: `67 tests`, `205 assertions`.
- Pint passed in the project container.

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Co-authored-by: Pascal HAVELANGE <havelangep@hotmail.com>