db restore staging and backup directories should use private permissions

[Widthdom]

Finding

db restore creates staging and backup directories next to the live database, then copies or moves DB/WAL/SHM files through them. These temporary directories can contain full database contents but are created through default Directory.CreateDirectory permissions.

Evidence

• src/CodeIndex/Cli/DbCommandRunner.cs:611 builds .restore-tmp-* and .restore-backup-* paths.
• src/CodeIndex/Cli/DbCommandRunner.cs:613 creates the restore temp directory.
• src/CodeIndex/Cli/DbCommandRunner.cs:622 creates the backup directory.
• The following moves and copies handle the live SQLite database and sidecars.

Suggested fix

Create restore staging and backup directories with owner-only permissions on POSIX, keep Windows behavior compatible, and add permission regression tests.

[Widthdom]

Work started on branch codex/fix-issue2868-2869-2879-2882.