feat: add Model Used section to PR template and checklist

Add a required "Model Used" section to the PR template so contributors
document which AI model (with version, context window, reasoning mode,
and other capability details) was used for each change. Also adds a
corresponding checklist item.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/PULL_REQUEST_TEMPLATE.md

@@ -38,9 +38,25 @@
 
 -
 
+## Model Used
+
+<!--
+  Required. Specify which AI model was used to produce or assist with
+  this change. Be as descriptive as possible — include:
+    • Provider and model name (e.g., Claude, GPT, Gemini, Codex)
+    • Exact model ID or version (e.g., claude-opus-4-6, gpt-4-turbo-2024-04-09)
+    • Context window size if relevant (e.g., 1M context)
+    • Reasoning/thinking mode if applicable (e.g., extended thinking, chain-of-thought)
+    • Any other relevant capability details (e.g., tool use, code execution)
+  If no AI model was used, write "None — human-authored".
+-->
+
+-
+
 ## Checklist
 
 - [ ] I have included a thinking path that traces from project context to this change
+- [ ] I have specified the model used (with version and capability details)
 - [ ] I have run tests locally and they pass
 - [ ] I have added or updated tests where applicable
 - [ ] If this change affects the UI, I have included before/after screenshots