`_read_jsonl` parsed every line with a bare `json.loads` inside a list
comprehension, so a single malformed line raised `JSONDecodeError` and
took the whole `load()` down. An append interrupted mid-write (process
crash, full disk) leaves exactly that: one truncated trailing line — and
from then on every surface that opens the session errors on load, with
no way back short of hand-editing the file. The session is effectively
bricked, including its recoverable history.
Skip unparseable lines and keep the good messages. This matches how the
rest of this module already treats JSON (the inline-blob and roots/grants
loaders all swallow `JSONDecodeError` and fall back) — `_read_jsonl` was
the one strict outlier on the hot session-load path.
