fix(restore): estimate tar size portably on macOS - #2922

Summary

estimate_restore_bytes_tar parsed column 3 of tar -tvz, which is the byte
size on GNU tar but the group field on BSD tar, so the pre-restore
disk-space guard silently became a no-op on macOS. The parser now takes
the first pure-numeric byte field, working on both GNU and BSD tar.

AI Assistance

AI helped draft; I reviewed every hunk and ran the checks below.

Release Lane

• Stable hotfix targeting release/2.6.x
• Mainline change targeting main
• Next-minor work targeting the next feature/minor release
• Not sure; reviewer should help classify

Changed Surface

• Docs only
• Tests only
• Dashboard UI
• Dashboard API / host agent
• Installer / bootstrap / lifecycle
• Docker Compose / service manifests
• Model routing / Hermes / capabilities
• Network exposure / auth / proxy
• Dependencies / runtime wiring

Risk And Validation

• Risk level: Medium
• Validation run:
  • git diff --check
  • Not required because: narrow change; syntax and diff checks pass locally

Commands/results:

git diff --check clean; bash -n on touched scripts passes

Operational Change Check

• This is an operational change and validation is recorded above.