docs: add admin provider setup task and update project guidelines
- add detailed task spec for /admin/providers UI with webhook integration
- add product sync strategy with manual, webhook, and scheduled sync
- update PROGRESS.md to prioritise admin provider UI as next task
- add writing style guidelines (british english, sentence case, concise)
- add commit guidelines (atomic, imperative, suggest at checkpoints)
- add pragmatic testing guidelines (test boundaries, skip trivial)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>