Implement unique filename generation to prevent overwriting existing session files

HOOK.md

@@ -17,6 +17,9 @@ metadata:
 Writes full user/assistant transcript context from the previous session into
 `<workspace>/memory/YYYY-MM-DD-slug.md` when `/new` or `/reset` runs.
 
+This hook avoids overwriting existing files by choosing a unique filename
+when a collision occurs (it appends `-1`, `-2`, etc. to the base name).
+
 Differences from bundled session-memory:
 
 - Uses full session context in output file