When /new rotates <session>.jsonl to <session>.jsonl.reset.*, the session-memory hook may read an empty active transcript and write header-only memory entries.
Add fallback logic to read the latest .jsonl.reset.* sibling when the primary file has no usable content.
Also add a unit test covering the rotated transcript path.
Fixes#18088
Refs #17563
Adds `messages` config option to session-memory hook (default: 15).
Fixes filter order bug - now filters user/assistant messages first,
then slices to get exactly N messages. Previously sliced first which
could result in fewer messages when non-message entries were present.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
