Find a session by its assigned name (not partial ID).
Iterates session headers one file at a time and returns on first match,
avoiding a full directory scan when the named session appears early.
The limit most-recently-modified sessions (by file mtime), most-recent
first. Sorts the directory entries by mtime WITHOUT parsing, then reads only
the top limit headers — so /sessions is O(limit) reads instead of
O(#sessions). mtime is an exact proxy for updated_at (the file is rewritten
on every save).