Repair stale Windows skill junctions during lifecycle commands

[gaoguobin]

Summary

• Rebuild stale or wrong-target Windows skill junctions during update.
• Let install remove stale reparse-point leftovers before creating the expected namespace link.
• Let uninstall remove broken junctions that Test-Path may miss.

Why

A Windows update can leave ~/.agents/skills/codex-environment-backup pointing at a deleted temporary path. Package install and doctor still pass, but Codex may fail to rescan the skill after restart.

Validation

• python -m unittest tests.test_docs -v
• python -m unittest discover -s tests
• python -m compileall src scripts
• git diff --check
• Manual PowerShell simulation of a stale junction repaired to the expected target