Add safe backfill for the public experiment archive

[YuanYuan98]

Historical backfill follow-up for #711. This PR is stacked on #859 and can remain draft until the team agrees on per-user source paths and permissions.

Goal

Copy existing successful and failed experiment run directories into the public archive after an authorized operator has reviewed them.

What changed

• add a dry-run-by-default backfill command to the archive helper from Publish team Torch experiments to OSN by default #859
• require an explicit source root and explicit --owner for each migration
• store every run under <owner>/<run-name> so identical run names from different users cannot collide
• scan only direct child run directories and support repeated --run selection
• refuse filesystem root, all of /scratch, and symlinked roots/runs
• use copy/check operations only; never sync or delete source or destination data
• continue across individual failures and optionally write a JSON migration report containing the owner
• document that migration runs once per authorized user root and requires source read permission

The default destination inherited from #859 is nyu-osn:m2lines-pubs/Samudra/experiments/<owner>/<run-name>.

Verification

• uv run pytest tests/test_experiment_archive.py (24 passed in the stacked tree)
• focused pre-commit hooks (all passed)
• uv run mypy scripts/experiment_archive.py
• Python 3.6 grammar check for the Torch host-side tool
• git diff --check

No historical data is moved by this PR itself. Running the migration remains a separate operator action after paths and permissions are agreed.