Add the append-only message log

Add the append-only message log

The log of one chat stores each message as one JSON line, {i, kind,
text, size, date}, in main/YYYY-MM-DD.jsonl for the local day of the
write. Each append goes to the disk before it returns, and the log loads
all lines again at start. An exclusive lock on the chat makes sure that
only one process writes it. After a failed write the log refuses more
writes, because the end of the file is not known.

The kinds are user, unii, tool, echo, work, and note. Size is the UTF-8
byte length of the text. Tests use testify, so the Nix package gets a
vendor hash.

Issue: beb9a70