docs: mark Step 3.6 as complete in PLAN.md

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PLAN.md

@@ -132,12 +132,12 @@ Check off items as completed. Each phase builds on the previous.
 - [x] **Commit checkpoint** (dc7700d)
 
 ### Step 3.6: Animal Tagging
-- [ ] Create migration for tag_suggestions table
-- [ ] Create `projections/tags.py` for tag intervals and suggestions
-- [ ] Update live_animals_by_location tags JSON
-- [ ] Add add_tag, end_tag to services/animal.py
-- [ ] Write tests: tag normalization, interval open/close, no-op handling
-- [ ] **Commit checkpoint**
+- [x] Create migration for tag_suggestions table
+- [x] Create `projections/tags.py` for tag intervals and suggestions
+- [x] Update live_animals_by_location tags JSON
+- [x] Add add_tag, end_tag to services/animal.py
+- [x] Write tests: tag normalization, interval open/close, no-op handling
+- [x] **Commit checkpoint** (0511ed7)
 
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