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>

PROGRESS.md

@@ -16,9 +16,12 @@
 
 ## Next Up
 
-1. Wire Products context to shop LiveViews (replace PreviewData)
-2. Add Printify product sync worker
-3. Session-based cart with real variants
+1. **Admin Provider Setup UI** (`/admin/providers`) - Add/edit/test provider connections
+2. **Product Sync Strategy:**
+   - ProductSyncWorker (Oban) for manual/scheduled sync
+   - Webhook endpoint for real-time updates from Printify
+3. Wire Products context to shop LiveViews (replace PreviewData)
+4. Session-based cart with real variants
 
 ---
 
@@ -56,11 +59,11 @@ See: [docs/plans/image-optimization.md](docs/plans/image-optimization.md) for im
 - [x] Product/variant/image schemas
 
 #### Remaining Tasks
-- [ ] Add `list_products/1` to Printify client (~1hr)
-- [ ] Create ProductSyncWorker (Oban) (~1hr)
+- [ ] **Admin Provider Setup UI** - `/admin/providers` for managing connections (~2hr) ← **NEXT**
+- [ ] Create ProductSyncWorker (Oban) for manual/scheduled sync (~1hr)
+- [ ] Printify webhook endpoint for real-time product updates (~1.5hr)
 - [ ] Wire shop LiveViews to Products context (~2hr)
 - [ ] Add variant selector component (~2hr)
-- [ ] Implement product sync mix task (~1hr)
 
 See: [docs/plans/products-context.md](docs/plans/products-context.md) for implementation details