v1.3.0

Changelog

• 9dd4b47: Open back-merge PR with a PAT so its checks run (@gabriel-ballesteros)
• e43e35d: Prepare release 1.3.0 (@gabriel-ballesteros)
• 8e415d3: docs(openspec): propose multi-pane workspace UI revamp (@gabriel-ballesteros)
• fb40205: docs(openspec): refine multi-pane workspace spec (@gabriel-ballesteros)
• e07429f: feat(ui): multi-pane workspace (@gabriel-ballesteros)
• a7ca29d: test(model): cover workspace panes; archive multi-pane change (@gabriel-ballesteros)