Add model selection to agent checklist

Adds checklist item prompting agent authors to make deliberate
model selection choices rather than defaulting to `inherit`.

Positioned after skills list, before capabilities.

Closes #19

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/writing-agents.md

@@ -577,6 +577,7 @@ Improve based on usage:
 - [ ] Name follows kebab-case convention
 - [ ] Agent has a clear, recognizable role
 - [ ] Skills list is deliberate (not too many, not too few)
+- [ ] Model selection is deliberate (not just `inherit` by default)
 - [ ] Capabilities are specific and achievable
 - [ ] "When to Use" guidance is clear
 - [ ] Behavioral rules prevent problems