Preserve opted-out workspace skills when shared sources are retired #81

What

Refresh can silently delete a customized workspace skill before its proposal merges.

Reproduce:

1. Opt out of an inherited skill, edit it, and leave the proposal unmerged.
2. Remove its shared source on main.
3. Remove the obsolete opt-out from the workspace and refresh.

Expected: preserve the customized copy or report a conflict. Actual: the copy is deleted.

Requirements

• Preserve workspace-owned edits when shared sources are retired.
• Report ambiguous ownership instead of silently deleting edits.
• Cover the unmerged-proposal case with a regression test.

Notes

Pre-existing bug reproduced on both the original base and #80.