docs: add explicit instruction to update task checkmarks in feature files

- Update Workflow section to emphasize marking tasks as [x] when completed
- Update Best Practices to explicitly state updating task checkmarks as you work

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

AGENTS.md

@@ -447,7 +447,7 @@ docs/features/
 ### Workflow
 1. **Create**: Copy `template.md` → `[feature-name].md`, fill in details
 2. **Plan**: Set status to `⏳ Planned`, add to `README.md` table
-3. **Develop**: Update status to `🚀 In Progress`, check off tasks
+3. **Develop**: Update status to `🚀 In Progress`, **mark tasks as `[x]` when completed**
 4. **Review**: Set status to `🔄 Code Review`, link PR in feature file
 5. **Complete**: Set status to `✅ Completed`, document lessons learned
 
@@ -460,6 +460,7 @@ docs/features/
 
 ### Best Practices
 - Create a feature file **before** starting development
+- **Update task checkmarks as you work** - Mark tasks as `[x]` when completed in the feature file
 - Update the file **as you work** (tasks, notes, decisions)
 - Be **specific** with tasks (not "implement X", but "create Y service", "add Z endpoint")
 - Document **design decisions** and **lessons learned**