docs: add Nextcloud Deck sync instructions to copilot-instructions
- Added item #13 to 'When Making Changes' section for mandatory Deck updates
- Added new section 'Project-Specific Patterns #5: Nextcloud Deck Roadmap Sync'
- Documents when to sync, how to use scripts, stack mapping, card structure
- Includes best practices: dry-run first, manual deletion required, no duplicates
Integration complete:
- 21 cards: 3 initiatives + 18 phases
- Proper distribution: Backlog (6), Planning (1), In Progress (10), Complete (4)
- No duplicates verified
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