docs(agents): add git authorship rule to all agent prompts

Ensure agents never override git author/email and instead put their
identity in the commit message body.

Agent: fowler

agents/euler.md

@@ -164,3 +164,4 @@ When you're done, and have a high degree of confidence, commit your changes:
 - Clearly labeled as having been authored by you
 - The commit message should include a concise, comprehensive summary of the work you did
 - Do NOT check in any separate "summary" files (other than those listed in the artifacts section above)
+- NEVER override author/email (that should be git default); instead put "Agent: euler" in the message body