sync: refresh plans after conflict decisions

[callumalpass]

Summary

• invalidate the reviewed plan before applying an explicit conflict decision
• obtain and render a fresh engine plan only after the decision succeeds
• keep a rejected stale decision from manufacturing a replacement preview

Found by live beta.41 Obsidian testing: resolution succeeded, but the view retained its pre-decision plan and left a stale apply button enabled.

Verification

• npm run build
• npm test (46 tests)
• live reproduced against the preserved production conflict fixture