Add agent co-authorship to spec front matter guidance

- Update specs.instructions.md with example showing AI agent in author field
- Update UserMessages spec front matter to include GitHub Copilot

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: denelon

.github/instructions/specs.instructions.md

@@ -25,6 +25,17 @@ issue id: <github issue id>
 ---
 ```
 
+If an AI agent (e.g., GitHub Copilot) assisted in authoring the spec, include it in the author field:
+
+```yaml
+---
+author: <name> <github-id>, GitHub Copilot <Copilot>
+created on: <yyyy-mm-dd>
+last updated: <yyyy-mm-dd>
+issue id: <github issue id>
+---
+```
+
 Update `last updated` whenever the spec is modified.
 
 ## Required Sections

doc/specs/#3483 - UserMessages.md

@@ -1,5 +1,5 @@
 ---
-author: Demitrius Nelon denelon
+author: Demitrius Nelon denelon, GitHub Copilot <Copilot>
 created on: 2026-05-03
 last updated: 2026-05-03
 issue id: 3483