feat: create typst skill from tutorial

.codex/skills/openspec-archive-change/SKILL.md

@@ -82,7 +82,17 @@ Archive a completed change in the experimental workflow.
    mv openspec/changes/<name> openspec/changes/archive/YYYY-MM-DD-<name>
    ```
 
-6. **Display summary**
+6. **Finalize synced spec purpose**
+
+   If specs were synced or created under `openspec/specs/`:
+   - Open each affected main spec at `openspec/specs/<capability>/spec.md`
+   - Check the `## Purpose` section
+   - If it contains the autogenerated archive placeholder (for example,
+     `TBD - created by archiving change ...`) or is still too generic, replace
+     it with a concise, capability-specific purpose statement before reporting
+     completion
+
+7. **Display summary**
 
    Show archive completion summary including:
    - Change name
@@ -112,3 +122,4 @@ All artifacts complete. All tasks complete.
 - Show clear summary of what happened
 - If sync is requested, use openspec-sync-specs approach (agent-driven)
 - If delta specs exist, always run the sync assessment and show the combined summary before prompting
+- If archiving creates or updates canonical specs, do not leave autogenerated placeholder `## Purpose` text behind