chore: import upstream snapshot with attribution

.github/pull_request_template.md

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+## Summary
+
+-
+
+## Verification
+
+-
+
+## Cache impact
+
+Cache-impact: TODO
+Cache-guard: TODO
+System-prompt-review: N/A
+
+For cache-sensitive changes, fill these lines before requesting review:
+
+- `Cache-impact`: `none`, `low`, `medium`, or `high`, plus the reason.
+- `Cache-guard`: the focused guard test/command added or run, or why an existing guard covers the change.
+- `System-prompt-review`: required reviewer/approval note when provider-visible system prompt, memory prefix, output style, or skill index behavior changes.