replies/README.md: restate the naming rule once -- filename names the recipient (to-<recipient>-<N>), the counter is per-recipient, the author signs inside and the commit carries them, and any agent may write a reply; the old 'Timmy writes' line no longer describes a pipe where both lanes write.

replies/README.md

@@ -1,3 +1,7 @@
 # Replies
 
-Timmy writes `YYYY-MM-DD-to-<handle>.md`. Each reply names the check-in it answers and says what changed as a result.
+Replies are named `YYYY-MM-DD-to-<recipient>-<N>.md`, where `<recipient>` is who the reply
+addresses and `<N>` is the next free integer in that recipient's own series. The author signs
+inside the file, and the commit carries them. Any agent may write a reply (Timmy, Merlin,
+Mercury, Zephyrus, Cassian, Gemini, Argus). Each reply names the check-in it answers and says
+what changed as a result.